Laser-diode-excited laser apparatus, fiber laser apparatus, and fiber laser amplifier in which laser medium doped with one of Ho3+, Sm3+, Eu3+, Dy3+, Er3+, and Tb3+ is excited with GaN-based compound laser diode

ABSTRACT

A solid-state laser crystal constituting a laser-diode-excited solid-state laser apparatus or an optical fiber constituting a fiber laser apparatus or fiber laser amplifier is doped with one of Ho 3+ , Sm 3+ , Eu 3+ , Dy 3+ , Er 3+ , and Tb 3+  so that a laser beam is emitted from the solid-state laser crystal or the optical fiber, or incident light of the fiber laser amplifier is amplified, by one of the transitions from  5 S 2  to  5 I 7 , from  5 S 2  to  5 I 8 , from  4 G 5/2  to  6 H 5/2 , from  4 G 5/2  to  6 H 7/2 , from  4 F 3/2  to  6 H 11/2 , from  5 D 0  to  7 F 2 , from  4 F 9/2  to  6 H 13/2 , from  4 F 9/2  to  6 H 11/2 , from  4 S 3/2  to  4 I 15/2 , from  2 H 9/2  to  4 I 13/2 , and from  5 D 4  to  7 F 5 . The above solid-state laser crystal or optical fiber is excited with a GaN-based compound laser diode.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The subject matters disclosed in this specification are relatedto the subject matters disclosed in the following copending,commonly-assigned U.S. patent applications:

[0002] (1) U.S. Pat. No. 6,125,132 filed by Yoji Okazaki (one of theapplicants of the present patent application) on Apr. 28, 1998 andentitled “LASER DIODE PUMPED SOLID STATE LASER, FIBER LASER AND FIBERAMPLIFIER,” corresponding to Japanese patent application Nos.10(1998)-6369 and 10(1998)-6370, which are laid open in JapaneseUnexamined Patent Publication Nos. 11(1999)-17266 and 11(1999)-204862;and

[0003] (2) U.S. Ser. No. 09/621,241 filed by Yoji Okazaki (one of theapplicants of the present patent application) and Takayuki Katoh(another of the applicants of the present patent application) on Jul.21, 2000 and entitled “LASER-DIODE-PUMPED LASER APPARATUS IN WHICHPr-DOPED LASER MEDIUM IS PUMPED WITH GaN-BASED COMPOUND LASER DIODE,”corresponding to Japanese patent application Nos. 11(1999)-206817 and11(1999)-206573, which are laid open in Japanese Unexamined PatentPublication Nos. 2001-36168 and 2001-36175.

[0004] The contents of the above copending, commonly-assigned U.S.patent applications (1) and (2) and the corresponding Japanese patentapplications are incorporated in this specification by reference.

BACKGROUND OF THE INVENTION

[0005] 1. Field of the Invention

[0006] The present invention relates to a laser-diode-excitedsolid-state laser apparatus in which a solid-state laser crystal dopedwith a rare-earth ion is excited with a laser diode (semiconductorlaser) so as to emit a laser beam.

[0007] The present invention also relates to a laser-diode-excitedsolid-state laser apparatus in which a solid-state laser crystal dopedwith a rare-earth ion is excited with a laser diode (semiconductorlaser), and which is arranged to emit ultraviolet light.

[0008] The present invention further relates to a laser-diode-excitedfiber laser apparatus in which a core of an optical fiber doped with arare-earth ion is excited with a laser diode (semiconductor laser) so asto emit a laser beam.

[0009] The present invention furthermore relates to alaser-diode-excited fiber laser amplifier in which a core of an opticalfiber doped with a rare-earth ion is excited with a laser diode(semiconductor laser) so as to amplify incident light by utilizingfluorescence generated by the excitation of the core.

[0010] 2. Description of the Related Art

[0011] (1) Solid-State Laser

[0012] Gas-laser-excited solid-state laser apparatuses in which a Pr³⁺-doped solid-state laser crystal is excited with a gas laser such asan Ar laser are known as disclosed in Journal of Applied Physics, vol.48, No. 2, 0 pp. 650-653 (1977) and Applied Physics, B58, pp. 149-151(1994). These solid-state laser apparatuses can generate a laser beam ina blue wavelength range of 470 to 490 nm by a transition from ³P₀ to ³H₄and a laser beam in a green wavelength range of 520 to 550 nm by atransition from ³P₁ to ³H₅. Therefore, the above solid-state laserapparatuses can be used as light sources for recording a color image ina color sensitive material.

[0013] In addition, another solid-state laser apparatus which emits alaser beam having a wavelength in the blue or green wavelength range isknown. For example, the Japanese Unexamined Patent Publication No.4(1992)318988 corresponding to the Japanese Patent Application No.3(1991)-086405, which is assigned to the present assignee, discloses alaser-diode-excited solid-state laser apparatus in which a solid-statelaser beam is converted into a second harmonic, i.e., the wavelength ofthe solid-state laser beam is reduced, by arranging a nonlinear opticalcrystal in a resonator.

[0014] Further, InGaN-based compound laser diodes and ZnMgSSe-basedcompound laser diodes which emit laser beams in the blue and greenwavelength ranges have recently been developed.

[0015] However, the light sources for use in recording a color image ina color image recording apparatus are required to be small in size,light in weight, and inexpensive. Nevertheless, the abovegas-laser-excited solid-state laser apparatus using the Pr³⁺-dopedsolid-state laser crystal is not suitable for use in recording a colorimage in a color image recording apparatus since the gas laser in thegas-laser-excited solid-state laser apparatus are large, heavy, andexpensive.

[0016] On the other hand, since the efficiency of wavelength conversionin the conventional laser-diode-excited solid-state laser apparatuses inwhich a wavelength of a solid-state laser beam is reduced by using anonlinear optical crystal is not sufficiently high, it is difficult toobtain high output power. In addition, in such laser-diode-excitedsolid-state laser apparatuses, an etalon or the like is inserted forlimiting the oscillation mode to a single mode. Therefore, loss in theresonator is great, and thus achievement of high output power becomesmore difficult.

[0017] Further, in order to match phases in the wavelength conversion inthe above laser-diode-excited solid-state laser apparatuses, highlyaccurate temperature control is required, and therefore the outputs ofthe laser-diode-excited solid-state laser apparatuses are not stable.Furthermore, the numbers of parts are increased by the provision of thenonlinear optical crystal and the etalon. Therefore, thelaser-diode-excited solid-state laser apparatuses become expensive.

[0018] When InGaN-based compound laser diodes are used, the oscillationwavelengths of the InGaN-based compound laser diodes increase withincrease in the indium contents, and theoretically it is possible toobtain laser beams in the blue wavelength range of 470 to 490 nm or inthe green wavelength range of 520 to 550 nm. However, since the qualityof the crystal deteriorates with the increase in the indium content, itis practically impossible to sufficiently increase the indium content,and the upper limit of the lengthened wavelength is about 450 nm.

[0019] In addition, blue light can be obtained by other laser diodeshaving an active layer made of an InGaNAs or GaNAs material. Theoscillation wavelengths in these laser diodes can also be increased bydoping the active layer with arsenic. However, since the quality of thecrystal also deteriorates with the increase in the arsenic content, theupper limit of the wavelength realizing high output power is about 450to 460 nm.

[0020] Further, the conventional ZnMgSSe-based compound laser diodescannot continuously oscillate at wavelengths below 500 nm at roomtemperature, and the lifetimes of the conventional ZnMgSSe-basedcompound laser diodes are at most about a hundred hours.

[0021] In order to solve the above problems, the copending,commonly-assigned U.S. Pat. No. 6,125,132 and the Japanese UnexaminedPatent Publication No. 11(1999)-17266 disclose a laser-diode-excitedsolid-state laser apparatus which is inexpensive, and can emit a laserbeam in the blue or green wavelength range with high efficiency, highoutput power, and high output stability. In this laser-diode-excitedsolid-state laser apparatus, a Pr³⁺-doped solid-state laser crystal isexcited with a GaN-based compound laser diode.

[0022] (2) Ultraviolet Laser

[0023] Highly efficient, high output power ultraviolet lasers whichcontinuously oscillate in the ultraviolet wavelength range are required,for example, for applications in ultraviolet lithography, fluorometricanalysis of organic cells using laser excitation, and the like.

[0024] GaN-based compound semiconductor lasers having an active layermade of an InGaN, InGaNAs, or GaNAs material are known as lasers whichoscillate in the ultraviolet wavelength range. Recently, GaN-basedcompound semiconductor lasers which can continuously oscillate for athousand hours at the wavelength of 400 nm with output power of severalmilliwatts have been provided.

[0025] On the other hand, wavelength-conversion solid-state lasers whichoutput ultraviolet laser beams having wavelengths of 400 nm or below areknown. In these wavelength-conversion solid-state lasers, wavelengths oflaser oscillation light are shortened to the ultraviolet wavelengths bysecond harmonic generation (SHG) or third harmonic generation (THG)using nonlinear optical crystals.

[0026] However, the conventional GaN-based compound semiconductor laserscannot emit laser light with output power of 100 mW or more in a singletransverse mode, although such laser light is required in manyapplications. In addition, the oscillation efficiency in theconventional GaN-based compound semiconductor lasers which emit laserlight having wavelengths of 380 nm or below is low, and the lifetimes ofsuch GaN-based compound semiconductor lasers are very short.

[0027] On the other hand, wavelength-conversion solid-state lasers whichoutput ultraviolet laser beams having wavelengths of 400 nm or below areknown. In these wavelength-conversion solid-state lasers, wavelengths oflaser oscillation light are shortened to the ultraviolet wavelengths bysecond harmonic generation (SHG) or third harmonic generation (THG)using nonlinear optical crystals.

[0028] However, solid-state laser mediums which realize efficientoscillation in the wavelength range of 700 to 800 nm have not yet beenfound. Therefore, it is difficult to obtain ultraviolet laser beams withhigh output power from the wavelength-conversion solid-state lasers inwhich the wavelengths of the laser light are shortened by secondharmonic generation (SHG).

[0029] In addition, the efficiency of the wavelength-conversionsolid-state lasers in which the wavelengths of the laser light areshortened by third harmonic generation (THG) is essentially low, and theconventional THG wavelength-conversion solid-state lasers can oscillatein only a pulse mode. In order to realize continuous oscillation, i.e.,in order to maintain resonance of THG light of the fundamental wave,highly accurate temperature adjustment of a resonator with a precisionof 0.01° C. is required. However, such accurate temperature adjustmentis practically difficult in terms of cost.

[0030] In order to solve the above problems, the copending,commonly-assigned U.S. Ser. No. 09/621,241 and the Japanese UnexaminedPatent Publication No. 200136175 disclose a laser-diode-excitedsolid-state laser apparatus in which a solid-state laser beam isconverted into a second harmonic by using an optical wavelengthconversion element so that ultraviolet light is obtained.

[0031] The above laser-diode-excited solid-state laser apparatuscomprises: a solid-state laser crystal which is doped with at least onerare-earth ion including at least Pr³⁺; a laser diode which has anactive layer made of one of InGaN, InGaNAs, and GaNAs materials, andemits an excitation laser beam for exciting the solid-state lasercrystal; and an optical wavelength conversion element which performswavelength conversion on a solid-state laser beam generated byexcitation of the solid-state laser crystal so as to generateultraviolet laser light.

[0032] Although the laser-diode-excited solid-state laser apparatusdisclosed in the U.S. Ser. No. 09/621,241 and the Japanese UnexaminedPatent Publication No. 2001-36175 can solve the aforementioned problems,the wavelength of the ultraviolet light which can be generated by thedisclosed laser-diode-excited solid-state laser apparatus is limited toabout 360 nm.

[0033] (3) Fiber Laser

[0034] As disclosed in the Technical Report of the Institute ofElectronics, information and Communication Engineers in Japan, LQE95-30(1995) p.30 and Optics Communications 86 (1991) p.337,laser-diode-excited fiber laser apparatuses which comprise a laser diodeand an optical fiber having a core made of a Pr³⁺-doped fluoride areknown. In the laser-diode-excited fiber laser apparatuses, the opticalfiber is excited with the laser diode so as to generate a laser beam.

[0035] In addition, as disclosed in the above references, optical fiberamplifiers which comprise a laser diode and an optical fiber having aPr³⁺-doped core are also known. In these optical fiber amplifiers, theoptical fiber is excited with the laser diode so that fluorescent lightis generated by the excitation of the optical fiber, and incident lightof the optical fiber is amplified by the energy of the fluorescent lightwhen the wavelength of the incident light is included in the wavelengthrange of the fluorescent light.

[0036] Further, Optics Communications 86 (1991) p.337 discloses anAr-laser-excited, Pr³⁺-doped fiber laser apparatuses, and laseroscillations at the wavelengths of 491, 520, 605, and 635 nm usingexcitation light having a wavelength of 476.5 nm have been reported.

[0037] The above laser-diode-excited fiber laser apparatuses andAr-laser-excited, Pr³⁺-doped fiber laser apparatuses can emit blue orgreen laser beams, and the above optical fiber amplifiers can amplifyblue or green laser beams. In this respect, it is considered that theseapparatuses and amplifiers can be used as constituents of light sourcesfor recording a color image in a color sensitive material.

[0038] However, in order to operate the above Ar-laser-excited,Pr³⁺-doped fiber laser apparatuses or Pr³⁺-doped fiber laser amplifierswith high power of a few watts to several tens of watts, for example,for recording a color image, a water cooling system is necessary.Therefore, the size is increased, and the lifetime and the efficiencyare reduced.

[0039] In order to solve the above problems, the copending,commonly-assigned U.S. Pat. No. 6,125,132 and the Japanese UnexaminedPatent Publication No. 11(1999)-204862 disclose a fiber laser apparatuswhich can efficiently emit a laser beam in a blue or green wavelengthrange with high output power and high stability in the output and beamquality, and can be formed in a small size. In this fiber laserapparatus, an optical fiber having a core doped with Pr³⁺ is excitedwith a GaN-based compound laser diode.

[0040] In addition, the U.S. Pat. No. 6,125,132 and the JapaneseUnexamined Patent Publication No. 11(1999)-204862 also disclose a fiberlaser amplifier which can efficiently amplify a laser beam in a blue orgreen wavelength range with high output power and high stability in theoutput and beam quality, and can be formed in a small size. In thisfiber laser amplifier, an optical fiber having a core doped with Pr³⁺ isexcited with a GaN-based compound laser diode so as to amplify incidentlight of the optical fiber when the wavelength of the incident light isin the wavelength range of fluorescent light generated by the excitationof the optical fiber.

[0041] Further, the U.S. Ser. No. 09/621,241 and the Japanese UnexaminedPatent Publication No. 2001-36168 disclose a fiber laser apparatus inwhich an optical fiber having a core codoped with Pr³⁺ and at least oneof Er³⁺, Ho³⁺, Dy³⁺, Eu³⁺, Sm³⁺, Pm³⁺, and Nd³⁺ is excited with aGaN-based compound laser diode. The U.S. Ser. No. 09/621,241 and theJapanese Unexamined Patent Publication No. 2001-36168 also disclose afiber laser amplifier in which an optical fiber having a core codopedwith Pr³⁺ and at least one of Er³⁺, Ho³⁺, Dy³⁺, Eu³⁺, Sm³⁺, Pm³⁺, andNd³⁺ is excited with a GaN-based compound laser diode so as to amplifyincident light of the optical fiber when the wavelength of the incidentlight is in the wavelength range of fluorescent light generated by theexcitation of the optical fiber.

SUMMARY OF THE INVENTION

[0042] A first object of the present invention is to provide alaser-diode-excited solid-state laser apparatus which uses a GaN-basedcompound laser diode as an excitation light source, and can emit laserlight in a wide wavelength range which is not covered by theconventional laser-diode-excited solid-state laser apparatuses whichuses a Pr³⁺-doped solid-state laser crystal.

[0043] A second object of the present invention is to provide alaser-diode-excited solid-state laser apparatus which is inexpensive,and can continuously emit ultraviolet light having a wavelength longeror shorter than 360 nm with high output power and high efficiency.

[0044] A third object of the present invention is to provide a fiberlaser apparatus which uses a GaN-based compound laser diode as anexcitation light source, and can emit laser light in a wide wavelengthrange which is not covered by the conventional fiber laser apparatuseswhich use a GaN-based compound laser diode as an excitation lightsource.

[0045] A fourth object of the present invention is to provide a fiberlaser amplifier which uses a GaN-based compound laser diode as anexcitation light source, and can amplify laser light in a widewavelength range which is not covered by the conventional fiber laseramplifiers which use a GaN-based compound laser diode as an excitationlight source.

[0046] (I) According to the first aspect of the present invention, thereis provided a laser-diode-excited solid-state laser apparatus including:a GaN-based compound laser diode which emits an excitation laser beam;and a solid-state laser crystal which is doped with Ho³⁺, and emits asolid-state laser beam generated by one of a first transition from ⁵S₂to ⁵I₇ and a second transition from ⁵S₂ to ⁵I₈ when the solid-statelaser crystal is excited with the excitation laser beam.

[0047] Preferably, the laser-diode-excited solid-state laser apparatusaccording to the first aspect of the present invention may also have oneor any possible combination of the following additional features (i) to(iii).

[0048] (i) The solid-state laser beam is generated by the firsttransition from ⁵S₂ to ⁵I₇ and is in the wavelength range of 740 to 760nm.

[0049] (ii) The solid-state laser beam is generated by the secondtransition from ⁵S₂ to ⁵I₈ and is in the wavelength range of 540 to 560nm.

[0050] (iii) The solid-state laser crystal is doped with no rare-earthion other than Ho³⁺.

[0051] The excitation wavelength of the solid-state laser crystal whichis doped with Ho³⁺ is 420 nm.

[0052] (II) According to the second aspect of the present invention,there is provided a laser-diode-excited solid-state laser apparatusincluding: a GaN-based compound laser diode which emits an excitationlaser beam; and a solid-state laser crystal which is doped with Sm³⁺,and emits a solid-state laser beam generated by one of a firsttransition from ⁴G_(5/2) to ⁶H_(5/ 2), a second transition from ⁴G_(5/2)to ⁶H_(7/2), and a third transition from ⁴F_(3/2) to ⁶H_(11/2) when thesolid-state laser crystal is excited with the excitation laser beam.

[0053] Preferably, the laser-diode-excited solid-state laser apparatusaccording to the second aspect of the present invention may also haveone or any possible combination of the following additional features (i)to (iv).

[0054] (i) The solid-state laser beam is generated by the firsttransition from ⁴G_(5/2) to ⁶H_(5/2) and is in the wavelength range of556 to 576 nm.

[0055] (ii) The solid-state laser beam is generated by the secondtransition from ⁴G_(5/2) to ⁶H_(7/2) and is in the wavelength range of605 to 625 nm.

[0056] (iii) The solid-state laser beam is generated by the thirdtransition from ⁴F_(3/2) to ⁶H_(11/2) and is in the wavelength range of640 to 660 nm.

[0057] (iv) The solid-state laser crystal is doped with no rare-earthion other than Sm³⁺.

[0058] The excitation wavelength of the solid-state laser crystal whichis doped with Sm³⁺ is 404 nm.

[0059] (III) According to the third aspect of the present invention,there is provided a laser-diode-excited solid-state laser apparatusincluding: a GaN-based compound laser diode which emits an excitationlaser beam; and a solid-state laser crystal which is doped with Eu³⁺ andemits a solid-state laser beam by a transition from ⁵D₀ to ⁷F₂ when thesolid-state laser crystal is excited with the excitation laser beam.

[0060] Preferably, the laser-diode-excited solid-state laser apparatusaccording to the third aspect of the present invention may also have oneor any possible combination of the following additional features (i) and(ii).

[0061] (i) The solid-state laser beam is in the wavelength range of 579to 599 nm.

[0062] (ii) The solid-state laser crystal is doped with no rare-earthion other than Eu³⁺.

[0063] The excitation wavelength of the solid-state laser crystal whichis doped with Eu³⁺ is 394 nm.

[0064] (IV) According to the fourth aspect of the present invention,there is provided a laser-diode-excited solid-state laser apparatusincluding: a GaN-based compound laser diode which emits an excitationlaser beam; and a solid-state laser crystal which is doped with Dy³⁺,and emits a solid-state laser beam generated by one of a firsttransition from ⁴F_(9/2) to ⁶H_(13/2) and a second transition from⁴F_(9/2) to ⁶H_(11/2) when the solid-state laser crystal is excited withthe excitation laser beam.

[0065] Preferably, the laser-diode-excited solid-state laser apparatusaccording to the fourth aspect of the present invention may also haveone or any possible combination of the following additional features (i)to (iii).

[0066] (i) The solid-state laser beam is generated by the firsttransition from F_(9/2) to ⁶H_(13/2) and is in the wavelength range of562 to 582 nm.

[0067] (ii) The solid-state laser beam is generated by the secondtransition from ⁴F_(9/2) to ⁶H_(11/2) and is in the wavelength range of654 to 674 nm.

[0068] (iii) The solid-state laser crystal is doped with no rare-earthion other than Dy³⁺.

[0069] The excitation wavelength of the solid-state laser crystal whichis doped with Dy³⁺ is 390 nm.

[0070] (V) According to the fifth aspect of the present invention, thereis provided a laser-diode-excited solid-state laser apparatus including:a GaN-based compound laser diode which emits an excitation laser beam;and a solid-state laser crystal which is doped with Er³⁺, and emits asolid-state laser beam generated by one of a first transition from⁴S_(3/2) to ⁴I_(15/2) and a second transition from ²H_(9/2) to ⁴I_(13/2)when the solid-state laser crystal is excited with the excitation laserbeam.

[0071] Preferably, the laser-diode-excited solid-state laser apparatusaccording to the fifth aspect of the present invention may also have oneor any possible combination of the following additional features (i) to(iii).

[0072] (i) The solid-state laser beam is generated by the firsttransition from ⁴S_(3/2) to ⁴I_(15/2) and is in the wavelength range of530 to 550 nm.

[0073] (ii) The solid-state laser beam is generated by the secondtransition from ²H_(9/2) to ⁴I_(13/2) and is in the wavelength range of544 to 564 nm.

[0074] (iii) The solid-state laser crystal is doped with no rare-earthion other than Er³⁺.

[0075] The excitation wavelength of the solid-state laser crystal whichis doped with Er³⁺ is 406 or 380 nm.

[0076] (VI) According to the sixth aspect of the present invention,there is provided a laser-diode-excited solid-state laser apparatusincluding: a GaN-based compound laser diode which emits an excitationlaser beam; and a solid-state laser crystal which is doped with Tb³⁺,and emits a solid-state laser beam generated by a transition from ⁵D₄ to⁷F₅ when the solid-state laser crystal is excited with the excitationlaser beam.

[0077] Preferably, the laser-diode-excited solid-state laser apparatusaccording to the sixth aspect of the present invention may also have oneor any possible combination of the following additional features (i) and(ii).

[0078] (i) The solid-state laser beam is in the wavelength range of 530to 550 nm.

[0079] (ii) The solid-state laser crystal is doped with no rare-earthion other than Tb³⁺.

[0080] The excitation wavelength of the solid-state laser crystal whichis doped with Tb³⁺ is 380 nm.

[0081] In addition, in the first to sixth aspects of the presentinvention, the GaN-based compound laser diode may have an active layermade of one of InGaN, InGaNAs, and GaNAs materials.

[0082] (VII) The advantages of the first to sixth aspects of the presentinvention are as follows.

[0083] (i) Since the rare-earth ions, Ho³⁺, Sm³⁺, Eu³⁺, Dy³⁺, Er³⁺, andTb³⁺ have their absorption bands in the wavelength range of 380 to 420nm, it is relatively easy to excite the rare-earth ions with a GaN-basedcompound laser diode. The wavelength range of 380 to 430 nm is awavelength range in which the GaN-based compound laser diodes canoscillate with relative ease. In particular, the currently availableGaN-based compound laser diodes can achieve their maximum output powerin the wavelength range of 400 to 410 nm. Therefore, when a solid-statelaser crystal doped with at least one of the rare-earth ions, Ho³⁺,Sm³⁺, Eu³⁺, Dy³⁺, Er³⁺, and Tb³⁺ is excited with a GaN-based compoundlaser diode, it is possible to make a great portion of the excitationlight absorbed by the solid-state laser crystal, and achieve highefficiency and high output power.

[0084] (ii) In addition, as individually exemplified before, thewavelength bands of the fluorescence generated by the excitation of thesolid-state laser crystals doped with the rare-earth ions, Ho³⁺, Sm³⁺,Eu³⁺, Dy³⁺, Er³⁺, and Tb³⁺ are distributed in a wide wavelength range.Therefore, it is possible to realize a laser-diode-excited solid-statelaser apparatus which can emit laser light having a wavelength which nolaser light capable of being generated by the conventionallaser-diode-excited solid-state laser apparatuses has.

[0085] (iii) On the other hand, the thermal conductivity coefficients ofthe GaN-based compound laser diodes are about 130 W/m° C., and muchgreater than the thermal conductivity coefficients of the ZnMgSSe-basedcompound laser diodes, which are about 4 W/m° C. In addition, since thedislocation mobility in the GaN-based compound laser diodes is very low,compared with that in the ZnMgSSe-based compound laser diodes, the COD(catastrophic optical damage) thresholds of the GaN-based compound laserdiodes are very high. Therefore, it is easy to obtain GaN-based compoundlaser diodes having a long lifetime and high output power. Since thelaser-diode-excited solid-state laser apparatuses according to the firstto sixth aspects of the present invention use a GaN-based compound laserdiode as an excitation light source, the laser-diode-excited solid-statelaser apparatuses according to the first to sixth aspects of the presentinvention can have a long lifetime, and emit a laser beam with highoutput power.

[0086] (VIII) According to the seventh aspect of the present invention,there is provided a laser-diode-excited solid-state laser apparatusincluding: a laser diode which has an active layer made of one of InGaN,InGaNAs, and GaNAs materials, and emits an excitation laser beam; asolid-state laser crystal which is doped with at least one rare-earthion including Ho³⁺, and emits a solid-state laser beam generated by oneof a first transition from ⁵S₂ to ⁵I₇ and a second transition from ⁵S₂to ⁵I₈ when the solid-state laser crystal is excited with the excitationlaser beam; and an optical wavelength conversion element which convertsthe solid-state laser beam into ultraviolet laser light by wavelengthconversion.

[0087] Preferably, the laser-diode-excited solid-state laser apparatusaccording to the seventh aspect of the present invention may also haveone or any possible combination of the following additional features (i)to (iii).

[0088] (i) The solid-state laser beam is generated by the firsttransition from ⁵S₂ to ⁵I₇ and has a wavelength of about 750 nm, and theultraviolet laser light has a wavelength of about 375 nm.

[0089] (ii) The solid-state laser beam is generated by the secondtransition from ⁵S₂ to ⁵I₈ and has a wavelength of about 550 nm, and theultraviolet laser light has a wavelength of about 275 nm.

[0090] (iii) The solid-state laser crystal is doped with no rare-earthion other than Ho³⁺.

[0091] The excitation wavelength of the solid-state laser crystal whichis doped with Ho³⁺ is 420 nm.

[0092] (IX) According to the eighth aspect of the present invention,there is provided a laser-diode-excited solid-state laser apparatusincluding: a laser diode which has an active layer made of one of InGaN,InGaNAs, and GaNAs materials, and emits an excitation laser beam; asolid-state laser crystal which is doped with at least one rare-earthion including Sm³⁺, and emits a solid-state laser beam generated by oneof a first transition from ⁴G_(5/2) to ⁶H_(5/2), a second transitionfrom ⁴G_(5/2) to ⁶H_(7/2), and a third transition from ⁴F_(3/2) to⁶H_(11/2) when the solid-state laser crystal is excited with theexcitation laser beam; and an optical wavelength conversion elementwhich converts the solid-state laser beam into ultraviolet laser lightby wavelength conversion.

[0093] Preferably, the laser-diode-excited solid-state laser apparatusaccording to the eighth aspect of the present invention may also haveone or any possible combination of the following additional features (i)to (iv).

[0094] (i) The solid-state laser beam is generated by the firsttransition from ⁴G_(5/2) to ⁶H_(5/2) and has a wavelength of about 566nm, and the ultraviolet laser light has a wavelength of about 283 nm.

[0095] (ii) The solid-state laser beam is generated by the secondtransition from ⁴G_(5/2) to H_(7/2) and has a wavelength of about 615nm, and the ultraviolet laser light has a wavelength of about 308 nm.

[0096] (iii) The solid-state laser beam is generated by the thirdtransition from ⁴F_(3/2) to ⁶H_(11/2) and has a wavelength of about 650nm, and the ultraviolet laser light has a wavelength of about 325 nm.

[0097] (iv) The solid-state laser crystal is doped with no rare-earthion other than Sm³⁺.

[0098] The excitation wavelength of the solid-state laser crystal whichis doped with Sm³⁺ is 404 nm.

[0099] (X) According to the ninth aspect of the present invention, thereis provided a laser-diode-excited solid-state laser apparatus including:a laser diode which has an active layer made of one of InGaN, InGaNAs,and GaNAs materials, and emits an excitation laser beam; a solid-statelaser crystal which is doped with at least one rare-earth ion includingEu³⁺, and emits a solid-state laser beam generated by a transition from⁵D₀ to ⁷F₂ when the solid-state laser crystal is excited with theexcitation laser beam; and an optical wavelength conversion elementwhich converts the solid-state laser beam into ultraviolet laser lightby wavelength conversion.

[0100] Preferably, the laser-diode-excited solid-state laser apparatusaccording to the ninth aspect of the present invention may also have oneor any possible combination of the following additional features (i) and(ii).

[0101] (i) The solid-state laser beam has a wavelength of about 589 nm,and the ultraviolet laser light has a wavelength of about 295 nm.

[0102] (ii) The solid-state laser crystal is doped with no rare-earthion other than Eu³⁺.

[0103] The excitation wavelength of the solid-state laser crystal whichis doped with Eu³⁺ is 394 nm.

[0104] (XI) According to the tenth aspect of the present invention,there is provided a laser-diode-excited solid-state laser apparatusincluding: a laser diode which has an active layer made of one of InGaN,InGaNAs, and GaNAs materials, and emits an excitation laser beam; asolid-state laser crystal which is doped with at least one rare-earthion including Dy³⁺, and emits a solid-state laser beam generated by oneof a first transition from ⁴F_(9/2) to ⁶H_(13/2) and a second transitionfrom ⁴F_(9/2) to ⁶H_(11/2) when the solid-state laser crystal is excitedwith the excitation laser beam; and an optical wavelength conversionelement which converts the solid-state laser beam into ultraviolet laserlight by wavelength conversion.

[0105] Preferably, the laser-diode-excited solid-state laser apparatusaccording to the tenth aspect of the present invention may also have oneor any possible combination of the following additional features (i) to(iii).

[0106] (i) The solid-state laser beam is generated by the firsttransition from ⁴F_(9/2) to ⁶H_(13/2) and has a wavelength of about 572nm, and the ultraviolet laser light has a wavelength of about 286 nm.

[0107] (ii) The solid-state laser beam is generated by the secondtransition from ⁴F_(9/2) to ⁶H_(11/2) and has a wavelength of about 664nm, and the ultraviolet laser light has a wavelength of about 332 nm.

[0108] (iii) The solid-state laser crystal is doped with no rare-earthion other than Dy³⁺.

[0109] The excitation wavelength of the solid-state laser crystal whichis doped with Dy³⁺ is 390 nm.

[0110] (XII) According to the eleventh aspect of the present invention,there is provided a laser-diodeexcited solid-state laser apparatusincluding: a laser diode which has an active layer made of one of InGaN,InGaNAs, and GaNAs materials, and emits an excitation laser beam; asolid-state laser crystal which is doped with at least one rare-earthion including Er³⁺, and emits a solid-state laser beam generated by oneof a first transition from ⁴S_(3/2) to ⁴I_(15/2) and a second transitionfrom ²H_(9/2) to ⁴I_(13/2) when the solid-state laser crystal is excitedwith the excitation laser beam; and an optical wavelength conversionelement which converts the solid-state laser beam into ultraviolet laserlight by wavelength conversion.

[0111] Preferably, the laser-diode-excited solid-state laser apparatusaccording to the eleventh aspect of the present invention may also haveone or any possible combination of the following additional features (i)to (iii).

[0112] (i) The solid-state laser beam is generated by the firsttransition from ⁴S_(3/2) to ⁴I_(15/2) and has a wavelength of about 540nm, and the ultraviolet laser light has a wavelength of about 270 nm.

[0113] (ii) The solid-state laser beam is generated by the secondtransition from ²H_(9/2) to ⁴I_(13/2) and has a wavelength of about 554nm, and the ultraviolet laser light has a wavelength of about 277 nm.

[0114] (iii) The solid-state laser crystal is doped with no rare-earthion other than Er³⁺.

[0115] The excitation wavelength of the solid-state laser crystal whichis doped with Er³⁺ is 406 or 380 nm.

[0116] In addition, in the seventh to eleventh aspects of the presentinvention, the optical wavelength conversion element may be realized bya nonlinear optical crystal having a periodic domain-inverted structure.

[0117] (XIII) The advantages of the seventh to eleventh aspects of thepresent invention are as follows.

[0118] (i) When a Ho³⁺-doped solid-state laser crystal, e.g., aHo³⁺-doped YAG crystal, is excited with a GaN-based compound laser diode(at an excitation wavelength of 420 nm), a solid-state laser beam in thenear infrared range is generated by a transition from ⁵S₂ to ⁵I⁷.Therefore, when this solid-state laser beam is converted into a secondharmonic by wavelength conversion using an optical wavelength conversionelement, ultraviolet light having a high intensity and a wavelengthlonger than 360 nm can be obtained. That is, a solid-state laser beamhaving a wavelength of, for example, about 750 nm can be obtained by theabove transition. Thus, when this solid-state laser beam is convertedinto a second harmonic, ultraviolet light having a high intensity and awavelength of about 375 nm is obtained.

[0119] (ii) In addition, when the Ho³⁺-doped solid-state laser crystalis excited with the GaN-based compound laser diode, a solid-state laserbeam having a wavelength of about 550 nm is generated by a transitionfrom ⁵S₂ to ⁵I₈. Therefore, when this solid-state laser beam isconverted into a second harmonic by wavelength conversion using anoptical wavelength conversion element, ultraviolet light having a highintensity and a wavelength of about 275 nm, which is shorter than 360nm, is obtained.

[0120] (iii) Although the construction for realizing the wavelengthconversion of the solid-state laser beam into a third harmonic iscomplex, the construction for realizing the wavelength conversion of thesolid-state laser beam into a second harmonic is simple. Therefore, thelaser-diode-excited solid-state laser apparatus using the wavelengthconversion into a second harmonic is inexpensive.

[0121] (iv) In the laser-diode-excited solid-state laser apparatusaccording to the eighth aspect of the present invention, in which aSm³⁺-doped solid-state laser crystal is used, solid-state laser beamshaving wavelengths of about 566, 615, and 650 nm can be generated byexcitation, for example, at the excitation wavelength of 404 nm, asexplained before. Therefore, when these solid-state laser beams areconverted into second harmonics by wavelength conversion, ultravioletlight beams having wavelengths of about 283, 308, and 325 nm can beobtained, respectively.

[0122] (v) In the laser-diode-excited solid-state laser apparatusaccording to the ninth aspect of the present invention, in which aEu³⁺-doped solid-state laser crystal is used, a solid-state laser beamhaving a wavelength of about 589 nm can be generated by excitation, forexample, at the excitation wavelength of 394 nm, as explained before.Therefore, when this solid-state laser beam is converted into a secondharmonic by wavelength conversion, an ultraviolet light beam having awavelength of about 295 nm can be obtained.

[0123] (vi) In the laser-diode-excited solid-state laser apparatusaccording to the tenth aspect of the present invention, in which aDy³⁺-doped solid-state laser crystal is used, solid-state laser beamshaving wavelengths of about 572 and 664 nm can be generated byexcitation, for example, at the excitation wavelength of 390 nm, asexplained before. Therefore, when these solid-state laser beams areconverted into second harmonics by wavelength conversion, ultravioletlight beams having wavelengths of about 286 and 332 nm can be obtained,respectively.

[0124] (vii) In the laser-diode-excited solid-state laser apparatusaccording to the eleventh aspect of the present invention, in which anEr³⁺-doped solid-state laser crystal is used, solid-state laser beamshaving wavelengths of about 540 and 554 nm can be generated byexcitation, for example, at the excitation wavelength of 406 or 380 nm,as explained before. Therefore, when these solid-state laser beams areconverted into second harmonics by wavelength conversion, ultravioletlight beams having wavelengths of about 270 and 277 nm can be obtained,respectively.

[0125] (viii) As explained before, the rare-earth ions, Ho³⁺, Sm³⁺,Eu³⁺, Dy³⁺ and Er³⁺ have their absorption bands in the wavelength rangeof 380 to 420 nm, in which the currently available GaN-based compoundlaser diodes can easily oscillate. In particular, the currentlyavailable GaN-based compound laser diodes can achieve their maximumoutput power in the wavelength range of 400 to 410 nm. Since thesolid-state laser crystals respectively doped with the rare-earth ions,Ho³⁺, Sm³⁺, Eu³⁺, Dy³⁺, and Er³⁺ are excited with a GaN-based compoundlaser diode in the laser-diode-excited solid-state laser apparatusesaccording to the seventh to eleventh aspects of the present invention, agreat portion of the excitation light is absorbed by the solid-statelaser crystal, and high efficiency and high output power can beachieved.

[0126] (ix) In addition, as explained before, the GaN-based compoundlaser diodes have a great thermal conductivity coefficient and a highCOD (catastrophic optical damage) threshold. Therefore, it is easy toobtain GaN-based compound laser diodes having a long lifetime and highoutput power. The laser-diode-excited solid-state laser apparatusesaccording to the seventh to eleventh aspects of the present invention,in which a GaN-based compound laser diode is used as an excitation lightsource, can have a long lifetime, and emit a laser beam with high outputpower.

[0127] (XIV) According to the twelfth aspect of the present invention,there is provided a fiber laser apparatus including: a GaN-basedcompound laser diode which emits a first laser beam; and an opticalfiber which has a core doped with Ho³⁺, and emits a second laser beamgenerated by one of a first transition from ⁵S₂ to ⁵I⁷ and a secondtransition from ⁵S₂ to ⁵I⁸ when the optical fiber is excited with thefirst laser beam.

[0128] Preferably, the fiber laser apparatus according to the twelfthaspect of the present invention may also have one or any possiblecombination of the following additional features (i) to (iii).

[0129] (i) The second laser beam is generated by the first transitionfrom ⁵S₂ to ⁵I₇ and is in the wavelength range of 740 to 760 nm.

[0130] (ii) The second laser beam is generated by the second transitionfrom ⁵S₂ to ⁵I₈ and is in the wavelength range of 540 to 560 nm.

[0131] (iii) The core of the optical fiber is doped with no rare-earthion other than Ho³⁺.

[0132] The excitation wavelength of the core of the optical fiber dopedwith Ho³⁺ is 420 nm.

[0133] (XV) According to the thirteenth aspect of the present invention,there is provided a fiber laser apparatus including: a GaN-basedcompound laser diode which emits a first laser beam; and an opticalfiber which has a core doped with Sm³⁺, and emits a second laser beamgenerated by one of a first transition from ⁴G_(5/2) to ⁶H_(5/2), asecond transition from ⁴G_(5/2) to ⁶H_(7/2), and a third transition from⁴F_(3/2) to ⁶H_(11/2) when the optical fiber is excited with the firstlaser beam.

[0134] Preferably, the fiber laser apparatus according to the thirteenthaspect of the present invention may also have one or any possiblecombination of the following additional features (i) to (iv).

[0135] (i) The second laser beam is generated by the first transitionfrom ⁴G_(5/2) to ⁶H_(5/2) and is in the wavelength range of 556 to 576nm.

[0136] (ii) The second laser beam is generated by the second transitionfrom ⁴G_(5/2) to ⁶H_(7/2) and is in the wavelength range of 605 to 625nm.

[0137] (iii) The second laser beam is generated by the third transitionfrom ⁴F_(3/2) to ⁶H_(11/2) and is in the wavelength range of 640 to 660nm.

[0138] (iv) The core of the optical fiber is doped with no rare-earthion other than Sm³⁺.

[0139] The excitation wavelength of the core of the optical fiber dopedwith Sm³⁺ is 404 nm.

[0140] (XVI) According to the fourteenth aspect of the presentinvention, there is provided a fiber laser apparatus including: aGaN-based compound laser diode which emits a first laser beam; and anoptical fiber which has a core doped with Eu³⁺, and emits a second laserbeam generated by a transition from ⁵D₀ to ⁷F₂ when the optical fiber isexcited with the first laser beam.

[0141] Preferably, the fiber laser apparatus according to the fourteenthaspect of the present invention may also have one or any possiblecombination of the following additional features (i) and (ii).

[0142] (i) The second laser beam is in the wavelength range of 579 to599 nm.

[0143] (ii) The core of the optical fiber is doped with no rare-earthion other than Eu³⁺.

[0144] The excitation wavelength of the core of the optical fiber dopedwith Eu³⁺ is 394 nm.

[0145] (XVII) According to the fifteenth aspect of the presentinvention, there is provided a fiber laser apparatus including: aGaN-based compound laser diode which emits a first laser beam; and anoptical fiber which has a core doped with Dy³⁺, and emits a second laserbeam generated by one of a first transition from ⁴F_(9/2) to ⁶H_(13/2)and a second transition from ⁴F_(9/2) to ⁶H_(11/2) when the opticalfiber is excited with the first laser beam.

[0146] Preferably, the fiber laser apparatus according to the fifteenthaspect of the present invention may also have one or any possiblecombination of the following additional features (i) to (iii).

[0147] (i) The second laser beam is generated by the first transitionfrom ⁴F_(9/2) to H_(13/2) and is in the wavelength range of 562 to 582nm.

[0148] (ii) The second laser beam is generated by the second transitionfrom ⁴F_(9/2) to ⁶H_(11/2) and is in the wavelength range of 654 to 674nm.

[0149] (iii) The core of the optical fiber is doped with no rare-earthion other than Dy³⁺.

[0150] The excitation wavelength of the core of the optical fiber dopedwith Dy³⁺ is 390 nm.

[0151] (XVIII) According to the sixteenth aspect of the presentinvention, there is provided a fiber laser apparatus including: aGaN-based compound laser diode which emits a first laser beam; and anoptical fiber which has a core doped with Er³⁺, and emits a second laserbeam generated by one of a first transition from ⁴S_(3/2) to ⁴I_(15/2)and a second transition from ²H_(9/2) to ⁴I_(13/2) when the opticalfiber is excited with the first laser beam.

[0152] Preferably, the fiber laser apparatus according to the sixteenthaspect of the present invention may also have one or any possiblecombination of the following additional features (i) to (iii).

[0153] (i) The second laser beam is generated by the first transitionfrom ⁴S_(3/2) to ⁴I_(15/2) and is in the wavelength range of 530 to 550nm.

[0154] (ii) The second laser beam is generated by the second transitionfrom ²H_(9/2) to ⁴I_(13/2) and is in the wavelength range of 544 to 564nm.

[0155] (iii) The core of the optical fiber is doped with no rare-earthion other than Er³⁺.

[0156] The excitation wavelength of the core of the optical fiber dopedwith Er³⁺ is 406 or 380 nm.

[0157] (XIX) According to the seventeenth aspect of the presentinvention, there is provided a fiber laser apparatus including: aGaN-based compound laser diode which emits a first laser beam; and anoptical fiber which has a core doped with Tb³⁺, and emits a second laserbeam generated by a transition from ⁵D₄ to ⁷F₅ when the optical fiber isexcited with the first laser beam.

[0158] Preferably, the fiber laser apparatus according to theseventeenth aspect of the present invention may also have one or anypossible combination of the following additional features (i) to (iii).

[0159] (i) The second laser beam is in the wavelength range of 530 to550 nm.

[0160] (ii) The core of the optical fiber is doped with no rare-earthion other than Tb³⁺.

[0161] The excitation wavelength of the core of the optical fiber dopedwith Tb³⁺ is 380 nm.

[0162] (XX) According to the eighteenth aspect of the present invention,there is provided a fiber laser amplifier including: a GaN-basedcompound laser diode which emits an excitation laser beam; and anoptical fiber which has a core doped with Ho³⁺, and amplifies incidentlight which has a wavelength within a wavelength range of fluorescencegenerated by one of a first transition from ⁵S₂ to ⁵I₇ and a secondtransition from ⁵S₂ to ⁵I₈ when the optical fiber is excited with theexcitation laser beam.

[0163] Preferably, the fiber laser amplifier according to the eighteenthaspect of the present invention may also have one or any possiblecombination of the following additional features (i) to (iii).

[0164] (i) The fluorescence is generated by the first transition from⁵S₂ to ⁵I₇ and is in the wavelength range of 740 to 760 nm.

[0165] (ii) The fluorescence is generated by the second transition from⁵S₂ to ⁵I₈ and is in the wavelength range of 540 to 560 nm.

[0166] (iii) The core of the optical fiber is doped with no rare-earthion other than Ho³⁺.

[0167] The excitation wavelength of the optical fiber which is dopedwith Ho³⁺ is 420 nm.

[0168] (XXI) According to the nineteenth aspect of the presentinvention, there is provided a fiber laser amplifier including: aGaN-based compound laser diode which emits an excitation laser beam; andan optical fiber which has a core doped with Sm³⁺, and amplifiesincident light which has a wavelength within a wavelength range offluorescence generated by one of a first transition from ⁴G_(5/2) to⁶H_(5/2), a second transition from ⁴G_(5/2) to ⁶H_(7/2), and a thirdtransition from ⁴F_(3/2) to ⁶H_(11/2) when the optical fiber is excitedwith the excitation laser beam.

[0169] Preferably, the fiber laser amplifier according to the nineteenthaspect of the present invention may also have one or any possiblecombination of the following additional features (i) to (iv).

[0170] (i) The fluorescence is generated by the first transition from⁴G_(5/2) to ⁶H_(5/2) and is in the wavelength range of 556 to 576 nm.

[0171] (ii) The fluorescence is generated by the second transition from⁴G_(5/2) to ⁶H_(7/2) and is in the wavelength range of 605 to 625 nm.

[0172] (iii) The fluorescence is generated by the third transition from⁴F_(3/2) to ⁶H_(11/2) and is in the wavelength range of 640 to 660 nm.

[0173] (iv) The core of the optical fiber is doped with no rare-earthion other than Sm³⁺.

[0174] The excitation wavelength of the core of the optical fiber dopedwith Sm³⁺ is 404 nm.

[0175] (XXII) According to the twentieth aspect of the presentinvention, there is provided a fiber laser amplifier including: aGaN-based compound laser diode which emits an excitation laser beam; andan optical fiber which has a core doped with Eu³⁺, and amplifiesincident light which has a wavelength within a wavelength range offluorescence generated by a transition from ⁵D₀ to ⁷F₂ when the opticalfiber is excited with the excitation laser beam.

[0176] Preferably, the fiber laser amplifier according to the twentiethaspect of the present invention may also have one or any possiblecombination of the following additional features (i) and (ii).

[0177] (i) The fluorescence is in the wavelength range of 579 to 599 nm.

[0178] (ii) The core of the optical fiber is doped with no rare-earthion other than Eu³⁺.

[0179] The excitation wavelength of the core of the optical fiber dopedwith Eu³⁺ is 394 nm.

[0180] (XXIII) According to the twenty-first aspect of the presentinvention, there is provided a fiber laser amplifier including: aGaN-based compound laser diode which emits an excitation laser beam; andan optical fiber which has a core doped with Dy³⁺, and amplifiesincident light which has a wavelength within a wavelength range offluorescence generated by one of a first transition from ⁴F_(9/2) to⁶H_(13/2) and a second transition from ⁴F_(9/2) to ⁶H_(11/2) when theoptical fiber is excited with the excitation laser beam.

[0181] Preferably, the fiber laser amplifier according to thetwenty-first aspect of the present invention may also have one or anypossible combination of the following additional features (i) to (iii).

[0182] (i) The fluorescence is generated by the first transition from⁴F_(9/2) to ⁶H_(13/2) and is in the wavelength range of 562 to 582 nm.

[0183] (ii) The fluorescence is generated by the second transition from⁴F_(9/2) to ⁶H_(11/2) and is in the wavelength range of 654 to 674 nm.

[0184] (iii) The core of the optical fiber is doped with no rare-earthion other than Dy³⁺.

[0185] The excitation wavelength of the optical fiber which is dopedwith Dy³⁺ is 390 nm.

[0186] (XXIV) According to the twenty-second aspect of the presentinvention, there is provided a fiber laser amplifier including: aGaN-based compound laser diode which emits an excitation laser beam; andan optical fiber which has a core doped with Er³⁺, and amplifiesincident light which has a wavelength within a wavelength range offluorescence generated by one of a first transition from ⁴S_(3/2) to⁴I_(15/2) and a second transition from ²H_(9/2) to ⁴I_(13/2) when theoptical fiber is excited with the excitation laser beam.

[0187] Preferably, the fiber laser amplifier according to thetwenty-second aspect of the present invention may also have one or anypossible combination of the following additional features (i) to (iii).

[0188] (i) The fluorescence is generated by the first transition from⁴S_(3/2) to ⁴I_(15/2) and is in the wavelength range of 530 to 550 nm.

[0189] (ii) The fluorescence is generated by the second transition from²H_(9/2) to ⁴I_(13/2) and is in the wavelength range of 544 to 564 nm.

[0190] (iii) The core of the optical fiber is doped with no rare-earthion other than Er³⁺.

[0191] The excitation wavelength of the core of the optical fiber dopedwith Er³⁺ is 406 or 380 nm.

[0192] (XXV) According to the twenty-third aspect of the presentinvention, there is provided a fiber laser amplifier including: aGaN-based compound laser diode which emits an excitation laser beam; andan optical fiber which has a core doped with Tb³⁺, and amplifiesincident light which has a wavelength within a wavelength range offluorescence generated by a transition from ⁵D₄ to ⁷F₅ when the opticalfiber is excited with the excitation laser beam.

[0193] Preferably, the fiber laser amplifier according to thetwenty-third aspect of the present invention may also have one or anypossible combination of the following additional features (i) and (ii).

[0194] (i) The fluorescence is in the wavelength range of 530 to 550 nm.

[0195] (ii) The core of the optical fiber is doped with no rare-earthion other than Tb³⁺.

[0196] The excitation wavelength of the core of the optical fiber dopedwith Tb³⁺ is 380 nm.

[0197] In addition, in the twelfth to twenty-third aspects of thepresent invention, the GaN-based compound laser diode may have an activelayer made of one of InGaN, InGaNAs, and GaNAs materials.

[0198] (XXVI) The advantages of the twelfth to twenty-third aspects ofthe present invention are as follows.

[0199] (i) For similar reasons to those explained in paragraph (VII)(i), in the fiber laser apparatuses and the fiber laser amplifiersaccording to the twelfth to seventeenth aspects of the presentinvention, a great portion of the excitation light is absorbed by theoptical fiber, and high efficiency and high output power can beachieved.

[0200] (ii) For similar reasons to those explained in paragraph (VII)(ii), the fiber laser apparatuses according to the twelfth toseventeenth aspects of the present invention can emit laser light havinga wavelength which no laser light capable of being generated by theconventional fiber laser apparatuses has, and the fiber laser amplifiersaccording to the eighteenth to twenty-third aspects of the presentinvention can amplify laser light having a wavelength which no laserlight capable of being amplified by the conventional fiber laseramplifiers has.

[0201] (iii) For the same reasons as those explained in paragraph (VII)(iii), the GaN-based compound laser diodes have a thermal conductivitycoefficient of about 130 W/m° C. , which is much greater than thethermal conductivity coefficient of the ZnMgSSe-based compound laserdiodes, which is about 4 W/m° C. . In addition, since the dislocationmobility in the GaN-based compound laser diodes is very low, comparedwith that in the ZnMgSSe-based compound laser diodes, the COD(catastrophic optical damage) thresholds of the GaN-based compound laserdiodes are very high. Therefore, it is easy to obtain GaN-based compoundlaser diodes having a long lifetime and high output power. Since thefiber laser apparatuses and the fiber laser amplifiers according to thetwelfth to twenty-third aspects of the present invention use a GaN-basedcompound laser diode as an excitation light source, thelaser-diode-excited solid-state laser apparatuses have a long lifetime,and can emit or amplify a laser beam with high output power.

[0202] (XXVII) In the constructions according to the first totwenty-third aspects of the present invention, the GaN-based compoundlaser diodes used as an excitation light source may be asingle-longitudinal-mode, single-transverse-mode, broad-area,phased-array, or MOPA (master oscillator power amplifier) type highpower laser diode. In addition, one or more GaN-based compound laserdiodes may be used in the constructions according to the first totwenty-third aspects of the present invention. In this case, theconstructions according to the first to seventeenth aspects of thepresent invention can emit a laser beam with further higher outputpower, e.g., on the order of 1 W.

DESCRIPTION OF THE DRAWINGS

[0203]FIG. 1 is a side view illustrating an outline of the constructionof a laser-diode-excited solid-state laser apparatus as a firstembodiment of the present invention.

[0204]FIG. 2 is a side view illustrating an outline of the constructionof a laser-diode-excited solid-state laser apparatus as a seventhembodiment of the present invention.

[0205]FIG. 3 is a side view illustrating an outline of the constructionof a fiber laser apparatus as a twelfth embodiment of the presentinvention.

[0206]FIG. 4 is a cross-sectional view of an optical fiber used in thefiber laser apparatus of FIG. 3.

[0207]FIG. 5 is a side view illustrating an outline of the constructionof a fiber laser amplifier as an eighteenth embodiment of the presentinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0208] Embodiments of the present invention are explained in detailbelow with reference to drawings.

First Embodiment

[0209] The first embodiment of the present invention is explained below.FIG. 1 is a side view illustrating an outline of the construction of alaser-diode-excited solid-state laser apparatus as the first embodimentof the present invention.

[0210] The laser-diode-excited solid-state laser apparatus of FIG. 1comprises a laser diode 11, a condenser lens 12, and a Ho³⁺-dopedsolid-state crystal 13. The laser diode 11 emits a laser beam 10 asexcitation light. The laser beam 10 is a divergent light beam, and thecondenser lens 12 condenses the laser beam 10. The Ho³⁺-dopedsolid-state crystal is an Y₃Al₅O₁₂ crystal doped with 1 atomic percent(atm %) of Ho³⁺, which is referred to as Ho:YAG crystal.

[0211] The above elements 11 to 13 are fixed on a Peltier element 14,and a thermistor 15 is attached to the Peltier element 14 for detectingtemperature. The output of the thermistor 15 is supplied to atemperature control circuit (not shown). Thus, the operation of thePeltier element 14 is controlled by the temperature control circuitbased on the output of the thermistor 15 so that the laser diode 11, thecondenser lens 12, and the Ho:YAG crystal 13 are maintained at apredetermined temperature.

[0212] The laser diode 11 is a broad-area type GaN-based compound laserdiode, which oscillates at the wavelength of 420 nm.

[0213] In order to generate a laser beam having the wavelength of 550 nmby the transition from ⁵S₂ to ⁵I₈ in the solid-state crystal 13, thebackward end surface (light entrance end face) 13 a of the solid-statecrystal 13 is coated to be highly reflective (HR) at the wavelength of550 nm and antireflective (AR) at other wavelengths including 750 nm(the wavelength of the fluorescence generated by the transition from ⁵S₂to ⁵I₇) and 420 nm (the wavelength of the laser beam 10). On the otherhand, the forward end surface 13 b of the solid-state crystal 13 iscoated so as to have a transmittance of 1% (i.e., a reflectance of 99%)at the wavelength of 550 nm.

[0214] The laser beam 10 being emitted from the laser diode 11 andhaving the wavelength of 420 nm enters the solid-state crystal 13through the backward end surface 13 a. In the solid-state crystal 13,Ho³⁺ is excited by the laser beam 10, and fluorescence having thewavelength of 550 nm is generated by the transition from ⁵S₂ to ⁵I₈ inthe solid-state crystal 13. The fluorescence having the wavelength of550 nm resonates between the forward and backward end surfaces 13 b and13 a, and causes a laser oscillation. Thus, a green laser beam 16 isgenerated in the solid-state crystal 13, and output through the forwardend surface 13 b.

[0215] In the laser-diode-excited solid-state laser apparatus as thefirst embodiment, the applicants have obtained the green laser beam 16with the output power of 100 mW when the output power of the GaN-basedcompound laser diode 11 is 300 mW.

[0216] Alternatively, the coatings applied to the forward and backwardend surfaces 13 b and 13 a may be arranged so that a laser beam havingthe wavelength of 750 nm is obtained from the solid-state crystal 13,since the solid-state crystal 13 can generate the fluorescence havingthe wavelength of 750 nm by the transition from ⁵S₂ to ⁵I₇.

Second Embodiment

[0217] The second embodiment of the present invention is explainedbelow. Since the construction of the laser-diode-excited solid-statelaser apparatus as the second embodiment of the present invention hasthe same construction as the first embodiment except for the portions ofthe construction explained below, the reference numerals in FIG. 1 arealso referred to in the following explanations of the second embodiment.

[0218] The laser-diode-excited solid-state laser apparatus as the secondembodiment is different from the laser-diode-excited solid-state laserapparatus as the first embodiment in the rare-earth ion with which thesolid-state crystal 13 is doped and the coatings applied to the forwardand backward end surfaces 13 b and 13 a.

[0219] That is, in the second embodiment, the solid-state crystal 13 isdoped with 1 atomic percent (atm %) of Sm³⁺ instead of Ho³⁺. Inaddition, the laser diode 11 used in the second embodiment emits a laserbeam having the wavelength of 404 nm. Further, in order to generate alaser beam having the wavelength of 566 nm by the transition from⁴G_(5/2) to ⁶H_(5/2) in the solid-state crystal 13, the backward endsurface (light entrance end face) 13 a of the solid-state crystal 13 iscoated to be highly reflective (HR) at the wavelength of 566 nm andantireflective (AR) at other wavelengths including 615 nm (thewavelength of the fluorescence generated by the transition from ⁴G_(5/2)to ⁶H_(7/2)), 650 nm (the wavelength of the fluorescence generated bythe transition from ⁴F_(3/2) to ⁶H_(11/2)), and 404 nm (the wavelengthof the excitation laser beam 10). On the other hand, the forward endsurface 13 b of the solid-state crystal 13 is coated so as to have atransmittance of 1% (i.e., a reflectance of 99%) at the wavelength of566 nm.

[0220] In the laser-diode-excited solid-state laser apparatus as thesecond embodiment, the applicants have obtained a laser beam 16 havingthe wavelength of 566 nm and the output power of 40 mW when the outputpower of the GaN-based compound laser diode 11 is 200 mW.

[0221] Alternatively, the coatings applied to the forward and backwardend surfaces 13 b and 13 a may be arranged so that a laser beam having awavelength of 615 or 650 nm is obtained from the Sm³⁺-doped solid-statecrystal 13, since the Sm³⁺-doped solid-state crystal 13 can generate thefluorescence having the wavelength of 615 nm by the transition from⁴G_(5/2) to ⁶H_(7/2) and the fluorescence having the wavelength of 650nm by the transition from ⁴F_(3/2) to ⁶H_(11/2).

[0222] For example, in the case where the laser beam having thewavelength of 615 nm is oscillated, it is possible to obtain asolid-state laser beam 16 having the output power of 50 mW by using aGaN-based compound laser diode 11 having the output power of 200 mW.

Third Embodiment

[0223] The third embodiment of the present invention is explained below.Since the construction of the laser-diode-excited solid-state laserapparatus as the third embodiment of the present invention also has thesame construction as the first embodiment except for the portions of theconstruction explained below, the reference numerals in FIG. 1 are alsoreferred to in the following explanations of the third embodiment.

[0224] The laser-diode-excited solid-state laser apparatus as the thirdembodiment is different from the laser-diode-excited solid-state laserapparatus as the first embodiment in the rare-earth ion with which thesolid-state crystal 13 is doped and the coatings applied to the forwardand backward end surfaces 13 b and 13 a.

[0225] That is, in the third embodiment, the solid-state crystal 13 isdoped with 1 atomic percent (atm %) of Eu³⁺ instead of Ho³⁺. Inaddition, the laser diode 11 used in the third embodiment emits a laserbeam having the wavelength of 394 nm. Further, in order to generate alaser beam having the wavelength of 589 nm by the transition from ⁵D₀ to⁷F₂ in the solid-state crystal 13, the backward end surface (lightentrance end face) 13 a of the solid-state crystal 13 is coated to behighly reflective (HR) at the wavelength of 589 nm and antireflective(AR) at other wavelengths including wavelengths of fluorescencegenerated by the other transitions and the excitation wavelength of 394nm. On the other hand, the forward end surface 13 b of the solid-statecrystal 13 is coated so as to have a transmittance of 1% (i.e., areflectance of 99%) at the wavelength of 589 nm.

[0226] In the laser-diode-excited solid-state laser apparatus as thethird embodiment, the applicants have obtained a laser beam 16 havingthe wavelength of 589 nm and the output power of 20 mW when the outputpower of the GaN-based compound laser diode 11 is 100 mW.

Fourth Embodiment

[0227] The fourth embodiment of the present invention is explainedbelow. Since the construction of the laser-diode-excited solid-statelaser apparatus as the fourth embodiment of the present invention hasthe same construction as the first embodiment except for the portions ofthe construction explained below, the reference numerals in FIG. 1 arealso referred to in the following explanations of the fourth embodiment.

[0228] The laser-diode-excited solid-state laser apparatus as the fourthembodiment is different from the laser-diode-excited solid-state laserapparatus as the first embodiment in the rare-earth ion with which thesolid-state crystal 13 is doped and the coatings applied to the forwardand backward end surfaces 13 b and 13 a.

[0229] That is, in the fourth embodiment, the solid-state crystal 13 isdoped with 1 atomic percent (atm %) of Dy³⁺ instead of Ho³⁺. Inaddition, the laser diode 11 used in the fourth embodiment emits a laserbeam having the wavelength of 390 nm. Further, in order to generate alaser beam having the wavelength of 572 nm by the transition from⁴F_(9/2) to ⁶H_(13/2) in the solid-state crystal 13, the backward endsurface (light entrance end face) 13 a of the solid-state crystal 13 iscoated to be highly reflective (HR) at the wavelength of 572 nm andantireflective (AR) at other wavelengths including 664 nm (thewavelength of the fluorescence generated by the transition from ⁴F_(9/2)to ⁶H_(11/2)) and 390 nm (the wavelength of the excitation laser beam10). On the other hand, the forward end surface 13 b of the solid-statecrystal 13 is coated so as to have a transmittance of 1% (i.e., areflectance of 99%) at the wavelength of 572 nm.

[0230] In the laser-diode-excited solid-state laser apparatus as thefourth embodiment, the applicants have obtained a laser beam 16 havingthe wavelength of 572 nm and the output power of 10 mW when the outputpower of the GaN-based compound laser diode 11 is 100 mW.

[0231] Alternatively, the coatings applied to the forward and backwardend surfaces 13 b and 13 a may be arranged so that a laser beam havingthe wavelength of 664 nm is obtained from the Dy³⁺-doped solid-statecrystal 13, since the Dy³⁺-doped solid-state crystal 13 can generate thefluorescence having the wavelength of 664 nm by the transition from⁴F_(9/2) to ⁶H_(11/2). In this case, it is possible to obtain asolid-state laser beam 16 having the output power of 10 mW by using aGaN-based compound laser diode 11 having the output power of 100 mW.

Fifth Embodiment

[0232] The fifth embodiment of the present invention is explained below.Since the construction of the laser-diode-excited solid-state laserapparatus as the fifth embodiment of the present invention has the sameconstruction as the first embodiment except for the portions of theconstruction explained below, the reference numerals in FIG. 1 are alsoreferred to in the following explanations of the fifth embodiment.

[0233] The laser-diode-excited solid-state laser apparatus as the fifthembodiment is different from the laser-diode-excited solid-state laserapparatus as the first embodiment in the rare-earth ion with which thesolid-state crystal 13 is doped and the coatings applied to the forwardand backward end surfaces 13 b and 13 a.

[0234] That is, in the fifth embodiment, the solid-state crystal 13 isdoped with 1 atomic percent (atm %) of Er³⁺ instead of Ho³⁺. Inaddition, the laser diode 11 used in the fifth embodiment emits a laserbeam having the wavelength of 406 nm. Further, in order to generate alaser beam having the wavelength of 554 nm by the transition from²H_(9/2) to ⁴I_(13/2) in the solid-state crystal 13, the backward endsurface (light entrance end face) 13 a of the solid-state crystal 13 iscoated to be highly reflective (HR) at the wavelength of 554 nm andantireflective (AR) at other wavelengths including 540 nm (thewavelength of the fluorescence generated by the transition from ⁴S_(3/2)to ⁴I_(15/2)) and 406 nm (the wavelength of the excitation laser beam10). On the other hand, the forward end surface 13 b of the solid-statecrystal 13 is coated so as to have a transmittance of 1% (i.e., areflectance of 99%) at the wavelength of 554 nm.

[0235] In the laser-diode-excited solid-state laser apparatus as thefifth embodiment, the applicants have obtained a laser beam 16 havingthe wavelength of 554 nm and the output power of 30 mW when the outputpower of the GaN-based compound laser diode 11 is 200 mW.

[0236] Alternatively, the coatings applied to the forward and backwardend surfaces 13 b and 13 a may be arranged so that a laser beam havingthe wavelength of 540 nm is obtained from the Er³⁺-doped solid-statecrystal 13, since the Er³⁺-doped solid-state crystal 13 can generate thefluorescence having the wavelength of 540 nm by the transition from⁴S_(3/2) to ⁴I_(15/2).

[0237] In addition, the excitation wavelength may be 380 nm instead of406 nm.

Sixth Embodiment

[0238] The sixth embodiment of the present invention is explained below.Since the construction of the laser-diode-excited solid-state laserapparatus as the sixth embodiment of the present invention also has thesame construction as the first embodiment except for the portions of theconstruction explained below, the reference numerals in FIG. 1 are alsoreferred to in the following explanations of the sixth embodiment.

[0239] The laser-diode-excited solid-state laser apparatus as the sixthembodiment is different from the laser-diode-excited solid-state laserapparatus as the first embodiment in the rare-earth ion with which thesolid-state crystal 13 is doped and the coatings applied to the forwardand backward end surfaces 13 b and 13 a.

[0240] That is, in the sixth embodiment, the solid-state crystal 13 isdoped with 1 atomic percent (atm %) of Tb³⁺ instead of Ho³⁺. Inaddition, the laser diode 11 used in the sixth embodiment emits a laserbeam having the wavelength of 380 nm. Further, in order to generate alaser beam having the wavelength of 540 nm by the transition from ⁵D₄ to⁷F₅ in the solid-state crystal 13, the backward end surface (lightentrance end face) 13 a of the solid-state crystal 13 is coated to behighly reflective (HR) at the wavelength of 540 nm and antireflective(AR) at other wavelengths including the wavelengths of the fluorescencegenerated by the other transitions and the excitation wavelength of 380nm. On the other hand, the forward end surface 13 b of the solid-statecrystal 13 is coated so as to have a transmittance of 1% (i.e., areflectance of 99%) at the wavelength of 540 nm.

[0241] In the laser-diode-excited solid-state laser apparatus as thesixth embodiment, the applicants have obtained a laser beam 16 havingthe wavelength of 540 nm and the output power of 10 mW when the outputpower of the GaN-based compound laser diode 11 is 100 mW.

Variations of First to Sixth Embodiments

[0242] Although the solid-state laser crystal 13 in the constructions ofthe first to sixth embodiments are an Y₃Al₅O₁₂ (YAG) crystal,alternatively, LiYF₄ (YLF), YVO₄, GdVO₄, BaY₂F₈, Ba(Y,Yb)₂F₈, LaF₃,Ca(NbO₃)₂, CaWO₄, SrMoO₄, YAlO₃ (YAP), Y₂SiO₅, YP₅O₁₄, LaP₅O₁₄, LuAlO₃,LaCl₃, LaBr₃, PrBr₃, or the like may be used.

[0243] The active layers of the laser diodes used as excitation lightsources in the constructions of the first to sixth embodiments can bemade of an InGaN-based, InGaNAs-based, or GaNAs-based compound material.In particular, when an absorption band of a solid-state laser crystal islocated on the longer wavelength side of the output wavelength of thelaser diode, it is preferable to use the InGaNAs-based or GaNAs-basedcompound material, since the wavelength of the laser oscillation in theInGaNAs-based or GaNAs-based compound laser diode can be lengthened moreeasily than that in the InGaN-based compound laser diode. That is, theabsorption efficiency in the InGaNAs-based or GaNAs-based compound laserdiode can be enhanced more easily than that in the InGaN-based compoundlaser diode.

Seventh Embodiment

[0244]FIG. 2 is a side view illustrating an outline of the constructionof a laser-diode-excited solid-state laser apparatus as the seventhembodiment of the present invention.

[0245] The laser-diode-excited solid-state laser apparatus of FIG. 2comprises a laser diode 111, a condenser lens 113, a solid-state lasermedium 114, a resonator mirror 115, an optical wavelength conversionelement 116, and an etalon 117. The laser diode 111 emits a laser beam110 as excitation light, where the laser beam 110 is a divergent lightbeam. The condenser lens 113 is realized by, for example, an indexdistribution type lens, and condenses the laser beam 110. Thesolid-state laser medium 114 is an Y₃Al₅O₁₂ crystal doped with 1 atomicpercent (atm %) of Ho³⁺, i.e., a Ho:YAG crystal. The resonator mirror115 is arranged on the forward side (i.e., on the right side in FIG. 2)of the Ho:YAG crystal 114. The optical wavelength conversion element 116and the etalon 117 are arranged between the Ho:YAG crystal 114 and theresonator mirror 115.

[0246] The above elements 114 to 117 are arranged on a common mount 130,which is made of, for example, copper. The mount 130 is fixed on aPeltier element 131, which constitutes a temperature adjustment means.The laser diode 111 and the condenser lens 113 are respectively arrangedon mounts 132 and 133, which are made of, for example, copper. Themounts 132 and 133 are also fixed on the Peltier element 131. ThePeltier element 131 is contained in a sealed case 136, which has a lightexit window 135.

[0247] In addition, a thermistor 134 is attached to the mount 130. Theoperation of the Peltier element 131 is controlled by a temperaturecontrol circuit (not shown) based on a temperature detection signalwhich is output from the thermistor 134, so that the laser diode 111 andall of the elements constituting a solid-state laser resonator aremaintained at a predetermined common temperature, where the solid-statelaser resonator is formed by the Ho:YAG crystal 114 and the resonatormirror 115 as explained later.

[0248] The optical wavelength conversion element 116 is produced byforming a periodic domain-inverted structure in a MgO-doped LiNbO₃crystal, which is a nonlinear optical material. In this example, theperiod of the periodic domain-inverted structure is 2.0 micrometers,which is the first order period with respect to the wavelength (750 nm)of the fundamental harmonic and the wavelength (375 nm) of the secondharmonic. The etalon 117 has a function of a wavelength selectionelement, and is provided for realizing oscillation of the solid-statelaser in a single longitudinal mode and reducing noise.

[0249] The laser diode 111 in the construction of FIG. 2 is a broad-areatype laser diode, which has an InGaN active layer and oscillates at thewavelength of 420 nm.

[0250] The backward end surface 114 a of the Ho:YAG crystal 114 is alight entrance surface, and is coated so as to have the followingtransmittance and reflectances. That is, the transmittance of thebackward end surface 114 a at the wavelength of 420 nm is 80% or higher,so that light having the wavelength of 420 nm efficiently transmitsthrough the backward end surface 114 a. In addition, the reflectance ofthe backward end surface 114 a at the wavelength of 750 nm is high,where the wavelength of 750 nm corresponds to one of the oscillationpeaks of Ho³⁺. For example, the reflectance of the backward end surface114 a at the wavelength of 750 nm is 99% or higher, and preferably 99.9%or higher. Further, the reflectances of the backward end surface 114 aat the wavelengths of the other oscillation peaks of Ho³⁺ (i.e., at thewavelengths of 550 nm, 980 nm, 1,010 nm, and 1,210 nm) are low. Forexample, the reflectances at the wavelengths of 550 nm, 980 nm, 1,010nm, and 1,210 nm are 60% or lower, and preferably 30% or lower.

[0251] On the other hand, the forward end surface 114 b of the Ho:YAGcrystal 114 is coated so as to have a low reflectance (e.g., 0.2% orlower) at the wavelength of 750 nm (the wavelength of the fundamentalharmonic) and a high reflectance (e.g., 95% or higher) at the wavelengthof 375 nm (the wavelength of the second harmonic).

[0252] The mirror surface 115 a of the resonator mirror 115 is coated soas to have a high reflectance (e.g., 99% or higher, and preferably 99.9%or higher) at the wavelength of 750 nm, a transmittance of 95% at thewavelength of 375 nm, and low reflectances (e.g., 60% or lower, andpreferably 30% or lower) at the wavelengths of the other oscillationpeaks including the wavelengths of 550 nm, 980 nm, 1,010 nm, and 1,210nm.

[0253] The laser beam 110 emitted from the laser diode 111 has thewavelength of 420 nm, and enters the Ho:YAG crystal 114 through thebackward end surface 114 a. Since Ho³⁺ in the Ho:YAG crystal 114 isexcited by the laser beam 110, the Ho:YAG crystal 114 generates lighthaving the wavelength of 750 nm by the transition from 5S2 to ⁵I₇. Then,laser oscillation at the wavelength of 750 nm occurs in a resonatorwhich is formed by the backward end surface 114 a of the Ho:YAG crystal114 and the mirror surface 115 a of the resonator mirror 115, and asolid-state laser beam 120 having the wavelength of 750 nm is generated.The solid-state laser beam 120 enters the optical wavelength conversionelement 116, and is converted to a second harmonic 121 having thewavelength of 375 nm, which is one-half of the wavelength of thesolid-state laser beam 120.

[0254] Since the mirror surface 115 a of the resonator mirror 115 iscoated as described before, only the second harmonic 121 exits throughthe resonator mirror 115. Thus, the second harmonic 121 exits from thesealed case 136 through the light exit window 135.

[0255] Since, in the laser-diode-excited solid-state laser apparatus asthe seventh embodiment of the present invention, the Ho:YAG crystal 114is excited with the InGaN laser diode, the efficiency and the outputpower are enhanced for the reason explained before. Actually, theapplicants have obtained the second harmonic 121 with the output powerof 40 mW when the output power of the laser diode 111 is 300 mW.

[0256] Although the operation explained above corresponds to theso-called continuous wave (CW) operation, the efficiency of thewavelength conversion can be enhanced by inserting a Q switch elementinto the resonator. In this case, the laser-diode-excited solid-statelaser apparatus operates in a pulse mode. Alternatively, pulsedultraviolet light can be obtained with high efficiency and high outputpower by driving the excitation laser diode in a pulse mode since theCOD (catastrophic optical damage) thresholds of the GaN-based compoundlaser diodes are high.

[0257] Further, the solid-state crystal 114 can generate a solid-statelaser beam having the wavelength of 550 nm by the transition from ⁵S₂ to⁵I₈. Therefore, when this solid-state laser beam is converted into asecond harmonic by using the optical wavelength conversion element 116,it is possible to obtain ultraviolet light having the wavelength of 275nm with a high intensity.

Eighth Embodiment

[0258] The construction of the laser-diode-excited solid-state laserapparatus as the eighth embodiment of the present invention also has thesame construction as the seventh embodiment except for the followingportions of the construction.

[0259] In the eighth embodiment, the solid-state laser crystal 114 isdoped with Sm³⁺ instead of Ho³⁺, and the laser beam 110 emitted from thelaser diode 111 has a wavelength of 404 nm, so that the solid-statelaser crystal 114 generates fluorescence having a wavelength of about566, 615, or 650 nm. In addition, the coatings applied to the backwardend surface 114 a and the forward end surface 114 b of the solid-statelaser crystal 114 and the mirror surface 115 a of the resonator mirror115 are arranged so that a solid-state laser beam having the wavelengthof about 566, 615, or 650 nm is generated in the resonator, a secondharmonic 121 having the wavelength of about 283, 308, or 325 nm isgenerated by the optical wavelength conversion element 116, andultraviolet light having the wavelength of about 283, 308, or 325 nm isoutput through the resonator mirror 115.

[0260] In the case where the Sm³⁺-doped solid-state laser crystal 114 isused, as well as the case where the Ho³⁺ doped solid-state laser crystal114 is used, particularly high output power can be obtained.

Ninth Embodiment

[0261] The construction of the laser-diode-excited solid-state laserapparatus as the ninth embodiment of the present invention also has thesame construction as the seventh embodiment except for the followingportions of the construction.

[0262] In the ninth embodiment, the solid-state laser crystal 114 isdoped with Eu³⁺ instead of Ho³⁺, and the laser beam 110 emitted from thelaser diode 111 has a wavelength of 394 nm, so that the solid-statelaser crystal 114 generates fluorescence having the wavelength of about589 nm. In addition, the coatings applied to the backward end surface114 a and the forward end surface 114 b of the solid-state laser crystal114 and the mirror surface 115 a of the resonator mirror 115 arearranged so that a solid-state laser beam having the wavelength of about589 nm is generated in the resonator, a second harmonic 121 having thewavelength of about 295 nm is generated by the optical wavelengthconversion element 116, and ultraviolet light having the wavelength ofabout 295 nm is output through the resonator mirror 115.

Tenth Embodiment

[0263] The construction of the laser-diode-excited solid-state laserapparatus as the tenth embodiment of the present invention also has thesame construction as the seventh embodiment except for the followingportions of the construction.

[0264] In the tenth embodiment, the solid-state laser crystal 114 isdoped with Dy³⁺ instead of Ho³⁺, and the laser beam 110 emitted from thelaser diode 111 has a wavelength of 390 nm, so that the solid-statelaser crystal 114 generates fluorescence having a wavelength of about572 or 664 nm. In addition, the coatings applied to the backward endsurface 114 a and the forward end surface 114 b of the solid-state lasercrystal 114 and the mirror surface 115 a of the resonator mirror 115 arearranged so that a solid-state laser beam having the wavelength of about572 or 664 nm is generated in the resonator, a second harmonic 121having the wavelength of about 286 or 332 nm is generated by the opticalwavelength conversion element 116, and ultraviolet light having thewavelength of about 286 or 332 nm is output through the resonator mirror115.

[0265] In the case where the Dy³⁺-doped solid-state laser crystal 114 isused, and the solid-state laser beam having the wavelength of 664 nm isgenerated, particularly high output power can be obtained.

Eleventh Embodiment

[0266] The construction of the laser-diode-excited solid-state laserapparatus as the eleventh embodiment of the present invention also hasthe same construction as the seventh embodiment except for the followingportions of the construction.

[0267] In the eleventh embodiment, the solid-state laser crystal 114 isdoped with Er³⁺ instead of Ho³⁺, and the laser beam 110 emitted from thelaser diode 111 has a wavelength of 406 or 380 nm, so that thesolid-state laser crystal 114 generates fluorescence having a wavelengthof about 540 or 554 nm. In addition, the coatings applied to thebackward end surface 114 a and the forward end surface 114 b of thesolid-state laser crystal 114 and the mirror surface 115 a of theresonator mirror 115 are arranged so that a solid-state laser beamhaving the wavelength of about 540 or 554 nm is generated in theresonator, a second harmonic 121 having the wavelength of about 270 or277 nm is generated by the optical wavelength conversion element 116,and ultraviolet light having the wavelength of about 270 or 277 nm isoutput through the resonator mirror 115.

Variations of Seventh to Eleventh Embodiments

[0268] Although the active layers of the laser diodes used as excitationlight sources in the constructions of the seventh to eleventhembodiments are made of InGaN, alternatively, the active layers of thelaser diodes may be made of an InGaNAs-based or GaNAs-based compoundmaterial.

[0269] Although the solid-state laser crystal 114 in the constructionsof the seventh to eleventh embodiments are an Y₃Al₅O₁₂ (YAG) crystal,alternatively, BaY₂F₈, Ba(Y,Yb)₂F₈, LaF₃, Ca(NbO₃)₂, CaWO₄, SrMoO₄,YAlO₃ (YAP), LiYF₄ (YLF), Y₂SiO₅, YP₅O₁₄, LaP₅O₁₄, LuAlO₃, LaCl₃, LaBr₃,PrBr₃, or the like may be used.

[0270] The period of the periodic domain-inverted structure in theoptical wavelength conversion element 116 may not be necessarily thefirst order period with respect to the wavelength of the fundamentalharmonic. Alternatively, the second or third order period may be used.For example, the third order period with respect to the wavelength of750 nm is 6.0 micrometers.

[0271] In addition, the optical wavelength conversion element 116 maynot be a type which has the periodic domain-inverted structure.Alternatively, the optical wavelength conversion element 116 may be atype which is made of B—BaBO₃, LBO, CLBO, GdYCOB, YCOB, or the like.

[0272] Further, the excitation laser diodes 111 may not be thebroad-area type. Alternatively, the laser diodes 111 may be a type whichincludes a MOPA (master oscillator power amplifier) or α-DFB(distributed feedback) structure.

Twelfth Embodiment

[0273]FIG. 3 is a side view illustrating an outline of the constructionof a fiber laser apparatus as the twelfth embodiment of the presentinvention.

[0274] The fiber laser apparatus of FIG. 3 comprises a laser diode 211,a condenser lens 212, and an optical fiber 213. The laser diode 211emits a laser beam 210 as excitation light, where the laser beam 210 isa divergent light beam. The condenser lens 212 condenses the laser beam210. The optical fiber 213 has a core 220 which is doped with Ho³⁺.

[0275] The laser diode 211 in the construction of FIG. 3 is a broad-areatype laser diode, which has an active layer made of a GaN-based compoundmaterial, and oscillates at the wavelength of 420 nm.

[0276]FIG. 4 shows a cross section of the optical fiber 213 used in thefiber laser apparatus of FIG. 3. As illustrated in FIG. 4, the opticalfiber 213 comprises the core 220 and first and second claddings 221 and222. The first cladding 221 is arranged around the core 220, and thesecond cladding 222 is arranged around the first cladding 221. Thecross-sectional shape of each of the core 220 and the second cladding222 is a true circle, and the cross-sectional shape of the firstcladding 221 is nearly a rectangle.

[0277] The core 220 is made of a zirconium-based fluoride glass, e.g.,ZBLANP (ZrF₄—BaF₂—LaF₃—AlF₃—AlF₃—NaF—PbF₂), and doped with 1 atomicpercent (atm %) of Ho³⁺. The first cladding 221 is made of, for example,ZBLAN (ZrF₄BaF₂—LaF₃—AlF₃—NaF), and the second cladding 222 is made of,for example, a polymer. Alternatively, the core 220 may be made of ZBLANor indium-gallium-based fluoride glass. For example, the core 220 may bemade of IGPZCL, i.e., (InF₃—GaF₃—LaF₃)-(PbF₂—ZnF₂)-CdF, or the like.

[0278] The laser beam 210 condensed by the condenser lens 212 enters thefirst cladding 221 of the optical fiber 213, and propagates through thefirst cladding 221 in a guided mode. That is, the first cladding 221behaves as a core for the laser beam 210. During the propagation, thelaser beam 210 also passes through the core 220. In the core 220, Ho³⁺is excited by the laser beam 210, so that fluorescence having thewavelength of 550 nm is generated by the transition from ⁵S₂ to ⁵I₈. Thefluorescence propagates through the core 220 in a guided mode.

[0279] In addition, the core 220 made of ZBLANP can also generate otherfluorescence such as the fluorescence having the wavelength of 750 nm,which is generated by the transition from ⁵S₂ to ⁵I₇. Therefore, thelight entrance end surface 213 a of the optical fiber 213 is coated tobe highly reflective (HR) at the wavelength of 550 nm, andantireflective (AR) at the excitation wavelength of 420 nm and thewavelengths of the fluorescence other than 550 nm. In addition, thelight exit end surface 213 b of the optical fiber 213 is coated so as totransmit only 1% of the light having the wavelength of 550 nm.

[0280] In the above configuration, the above fluorescence having thewavelength of 550 nm resonates between the light entrance end surface213 a and the light exit end surface 213 b of the optical fiber 213,i.e., laser oscillation occurs at the wavelength of 550 nm. Thus, agreen laser beam 215 having the wavelength of 550 nm is generated in theoptical fiber 213, and exits from the light exit end surface 213 b ofthe optical fiber 213 to the forward side of the fiber laser apparatusof FIG. 3.

[0281] In this example, the laser beam 215 propagates through the core220 in a single mode, and the laser beam 210 propagates through thefirst cladding 221 in multiple modes. Therefore, it is possible to use ahigh-power, broad-area type laser diode as the excitation light source,and input the laser beam 210 from the high-power, broad-area type laserdiode into the optical fiber 213 with high coupling efficiency.

[0282] In addition, since the cross-sectional shape of the firstcladding 221 is nearly a rectangle, the laser beam 210 propagatesthrough irregular reflection paths within the first cladding 221, andtherefore the probability of entrance of the laser beam 210 into thecore 220 is enhanced.

[0283] Further, since the wavelength 420 nm of the laser beam 210 iswithin the wavelength range in which the output power of the GaN-basedcompound laser diodes is enhanced, the amount of the laser beam 210absorbed by the optical fiber 213 becomes great, and high efficiency andhigh output power can be achieved. Actually, the applicants haveobtained the green laser beam 215 with the output power of 150 mW whenthe output power of the laser diode 211 is 300 mW, and the length of theoptical fiber 213 is 1 m.

[0284] Furthermore, the Ho³⁺-doped core 220 of the optical fiber 213 cangenerate the fluorescence having the wavelength of 750 nm by thetransition from ⁵S₂ to ⁵I₇. Therefore, when the coatings applied to thelight entrance end surface 213 a and the light exit end surface 213 b ofthe optical fiber 213 are appropriately arranged, the fiber laserapparatus of FIG. 3 can emit a laser beam having the wavelength of 750nm.

Thirteenth Embodiment

[0285] The thirteenth embodiment of the present invention is explainedbelow. Since the construction of the fiber laser apparatus as thethirteenth embodiment of the present invention has the same constructionas the twelfth embodiment except for the portions of the constructionexplained below, the reference numerals in FIG. 3 are also referred toin the following explanations of the thirteenth embodiment.

[0286] The fiber laser apparatus as the thirteenth embodiment isdifferent from the fiber laser apparatus as the twelfth embodiment inthe rare-earth ion with which the core 220 of the optical fiber 213 isdoped and the coatings applied to the light entrance end surface 213 aand the light exit end surface 213 b of the optical fiber 213.

[0287] That is, in the thirteenth embodiment, the core 220 of theoptical fiber 213 is doped with 1 atomic percent (atm %) of Sm³⁺ insteadof Ho³⁺. In addition, the laser diode 211 used in the thirteenthembodiment emits a laser beam having the wavelength of 404 nm. Further,in order to generate a laser beam having the wavelength of 566 nm by thetransition from ⁴G_(5/2) to ⁶H_(5/2) in the core 220 of the opticalfiber 213, the light entrance end surface 213 a of the optical fiber 213is coated to be highly reflective (HR) at the wavelength of 566 nm andantireflective (AR) at other wavelengths including 615 nm (thewavelength of the fluorescence generated by the transition from ⁴G_(5/2)to ⁶H_(7/2)), 650 nm (the wavelength of the fluorescence generated bythe transition from ⁴F_(3/2) to ⁶H_(11/2)), and 404 nm (the wavelengthof the excitation laser beam 210). On the other hand, the light exit endsurface 213 b of the optical fiber 213 is coated so as to have atransmittance of 1% (i.e., a reflectance of 99%) at the wavelength of566 nm.

[0288] In the fiber laser apparatus as the thirteenth embodiment, theapplicants have obtained a laser beam 215 having the wavelength of 566nm and the output power of 110 mW when the output power of the GaN-basedcompound laser diode 211 is 200 mW, and the length of the optical fiber213 is 1 m.

[0289] Alternatively, the coatings applied to the light entrance endsurface 213 a and the light exit end surface 213 b of the optical fiber213 may be arranged so that a laser beam having a wavelength of 615 or650 nm is generated in the optical fiber 213, since the Sm³⁺-doped core220 of the optical fiber 213 can generate the fluorescence having thewavelength of 615 nm by the transition from ⁴G_(5/2) to ⁶H_(7/2) and thefluorescence having the wavelength of 650 nm by the transition from⁴F_(3/2) to ⁶H_(11/2).

Fourteenth Embodiment

[0290] The fourteenth embodiment of the present invention is explainedbelow. Since the construction of the fiber laser apparatus as thefourteenth embodiment of the present invention also has the sameconstruction as the twelfth embodiment except for the portions of theconstruction explained below, the reference numerals in FIG. 3 are alsoreferred to in the following explanations of the fourteenth embodiment.

[0291] The fiber laser apparatus as the fourteenth embodiment isdifferent from the fiber laser apparatus as the twelfth embodiment inthe rare-earth ion with which the core 220 of the optical fiber 213 isdoped and the coatings applied to the light entrance end surface 213 aand the light exit end surface 213 b of the optical fiber 213.

[0292] That is, in the fourteenth embodiment, the core 220 of theoptical fiber 213 is doped with 1 atomic percent (atm %) of Eu³⁺ insteadof Ho³⁺. In addition, the laser diode 211 used in the fourteenthembodiment emits a laser beam having the wavelength of 394 nm. Further,in order to generate a laser beam having the wavelength of 589 nm by thetransition from ⁵D₀ to ⁷F₂ in the core 220 of the optical fiber 213, thelight entrance end surface 213 a of the optical fiber 213 is coated tobe highly reflective (HR) at the wavelength of 589 nm and antireflective(AR) at other wavelengths including the wavelengths of the fluorescencegenerated by the other transitions and the excitation wavelength of 394nm. On the other hand, the light exit end surface 213 b of the opticalfiber 213 is coated so as to have a transmittance of 1% (i.e., areflectance of 99%) at the wavelength of 589 nm.

[0293] In the fiber laser apparatus as the fourteenth embodiment, theapplicants have obtained a laser beam 215 having the wavelength of 589nm and the output power of 40 mW when the output power of the GaN-basedcompound laser diode 211 is 100 mW, and the length of the optical fiber213 is 1 m.

Fifteenth Embodiment

[0294] The fifteenth embodiment of the present invention is explainedbelow. Since the construction of the fiber laser apparatus as thefifteenth embodiment of the present invention has the same constructionas the twelfth embodiment except for the portions of the constructionexplained below, the reference numerals in FIG. 3 are also referred toin the following explanations of the fifteenth embodiment.

[0295] The fiber laser apparatus as the fifteenth embodiment isdifferent from the fiber laser apparatus as the twelfth embodiment inthe rare-earth ion with which the core 220 of the optical fiber 213 isdoped and the coatings applied to the light entrance end surface 213 aand the light exit end surface 213 b of the optical fiber 213.

[0296] That is, in the fifteenth embodiment, the core 220 of the opticalfiber 213 is doped with 1 atomic percent (atm %) of Dy³⁺ instead ofHo³⁺. In addition, the laser diode 211 used in the fifteenth embodimentemits a laser beam having the wavelength of 390 nm. Further, in order togenerate a laser beam having the wavelength of 572 nm by the transitionfrom ⁴F₉ /2 to ⁶H_(13/2) in the core 220 of the optical fiber 213, thelight entrance end surface 213 a of the optical fiber 213 is coated tobe highly reflective (HR) at the wavelength of 572 nm and antireflective(AR) at other wavelengths including 664 nm (the wavelength of thefluorescence generated by the transition from ⁴F_(9/2) to ⁶H_(11/2)) and390 nm (the wavelength of the excitation laser beam 210). On the otherhand, the light exit end surface 213 b of the optical fiber 213 iscoated so as to have a transmittance of 1% (i.e., a reflectance of 99%)at the wavelength of 572 nm.

[0297] In the fiber laser apparatus as the fifteenth embodiment, theapplicants have obtained a laser beam 215 having the wavelength of 572nm and the output power of 50 mW when the output power of the GaN-basedcompound laser diode 211 is 100 mW, and the length of the optical fiber213 is 1 m.

[0298] Alternatively, the coatings applied to the light entrance endsurface 213 a and the light exit end surface 213 b of the optical fiber213 may be arranged so that a laser beam having the wavelength of 664 nmis generated in the optical fiber 213, since the Dy³⁺-doped core 220 ofthe optical fiber 213 can generate the fluorescence having thewavelength of 664 nm by the transition from ⁴F_(9/2) to ⁶H_(11/2).

Sixteenth Embodiment

[0299] The sixteenth embodiment of the present invention is explainedbelow. Since the construction of the fiber laser apparatus as thesixteenth embodiment of the present invention has the same constructionas the twelfth embodiment except for the portions of the constructionexplained below, the reference numerals in FIG. 3 are also referred toin the following explanations of the sixteenth embodiment.

[0300] The fiber laser apparatus as the sixteenth embodiment isdifferent from the fiber laser apparatus as the twelfth embodiment inthe rare-earth ion with which the core 220 of the optical fiber 213 isdoped and the coatings applied to the light entrance end surface 213 aand the light exit end surface 213 b of the optical fiber 213.

[0301] That is, in the sixteenth embodiment, the core 220 of the opticalfiber 213 is doped with 1 atomic percent (atm %) of Er³⁺ instead ofHo³⁺. In addition, the laser diode 211 used in the sixteenth embodimentemits a laser beam having the wavelength of 406 nm. Further, in order togenerate a laser beam having the wavelength of 554 nm by the transitionfrom ²H_(9/2) to ⁴I_(13/2) in the core 220 of the optical fiber 213, thelight entrance end surface 213 a of the optical fiber 213 is coated tobe highly reflective (HR) at the wavelength of 554 nm and antireflective(AR) at other wavelengths including 540 nm (the wavelength of thefluorescence generated by the transition from ⁴S_(3/2) to ⁴I_(15/2)) and406 nm (the wavelength of the excitation laser beam 210). On the otherhand, the light exit end surface 213 b of the optical fiber 213 iscoated so as to have a transmittance of 1% (i.e., a reflectance of 99%)at the wavelength of 554 nm.

[0302] In the fiber laser apparatus as the sixteenth embodiment, theapplicants have obtained a laser beam 215 having the wavelength of 554nm and the output power of 120 mW when the output power of the GaN-basedcompound laser diode 211 is 200 mW, and the length of the optical fiber213 is 1 m.

[0303] Alternatively, the coatings applied to the light entrance endsurface 213 a and the light exit end surface 213 b of the optical fiber213 may be arranged so that a laser beam having the wavelength of 540 nmis generated in the optical fiber 213, since the Er³⁺-doped core 220 ofthe optical fiber 213 can generate the fluorescence having thewavelength of 540 nm by the transition from ⁴S_(3/2) to ⁴I_(15/2).

[0304] In addition, the excitation wavelength may be 380 nm instead of406 nm.

Seventeenth Embodiment

[0305] The seventeenth embodiment of the present invention is explainedbelow. Since the construction of the fiber laser apparatus as theseventeenth embodiment of the present invention also has the sameconstruction as the twelfth embodiment except for the portions of theconstruction explained below, the reference numerals in FIG. 3 are alsoreferred to in the following explanations of the seventeenth embodiment.

[0306] The fiber laser apparatus as the seventeenth embodiment isdifferent from the fiber laser apparatus as the twelfth embodiment inthe rare-earth ion with which the core 220 of the optical fiber 213 isdoped and the coatings applied to the light entrance end surface 213 aand the light exit end surface 213 b of the optical fiber 213.

[0307] That is, in the seventeenth embodiment, the core 220 of theoptical fiber 213 is doped with 1 atomic percent (atm %) of Tb³⁺ insteadof Ho³⁺. In addition, the laser diode 211 used in the seventeenthembodiment emits a laser beam having the wavelength of 380 nm. Further,in order to generate a laser beam having the wavelength of 540 nm by thetransition from ⁵D₄ to ⁷F₅ in the core 220 of the optical fiber 213, thelight entrance end surface 213 a of the optical fiber 213 is coated tobe highly reflective (HR) at the wavelength of 540 nm and antireflective(AR) at other wavelengths including the wavelengths of the fluorescencegenerated by the other transitions and the excitation wavelength of 380nm. On the other hand, the light exit end surface 213 b of the opticalfiber 213 is coated so as to have a transmittance of 1% (i.e., areflectance of 99%) at the wavelength of 540 nm.

[0308] In the fiber laser apparatus as the seventeenth embodiment, theapplicants have obtained a laser beam 215 having the wavelength of 540nm and the output power of 30 mW when the output power of the GaN-basedcompound laser diode 211 is 100 mW, and the length of the optical fiber213 is 1 m.

Eighteenth Embodiment

[0309]FIG. 5 is a side view illustrating an outline of the constructionof a fiber laser amplifier as the eighteenth embodiment of the presentinvention. In FIG. 5, elements having the same reference numbers as FIG.3 have the same functions as the corresponding elements in FIG. 3.

[0310] The fiber laser amplifier of FIG. 5 comprises a laser diode 211,a collimator lens 250, a condenser lens 251, a beam splitter 252, anoptical fiber 253, an SHG (second harmonic generation) laser unit 256,and another collimator lens 257. The laser diode 211 emits a laser beam210 having the wavelength of 420 nm as excitation light, where the laserbeam 210 is a divergent light beam. The collimator lens 250 collimatesthe laser beam 210. The beam splitter 252 is arranged between thecollimator lens 250 and the condenser lens 251, and the laser beam 210collimated by the collimator lens 250 passes through the beam splitter252. The condenser lens 251 condenses the collimated laser beam 210, andthe condensed laser beam 210 enters the optical fiber 253, which has aHo³⁺-doped core.

[0311] The SHG laser unit 256 is provided for emitting a laser beam 255having the wavelength of 550 nm. Although not shown, the SHG laser unit256 includes a DBR (distributed Bragg reflection) type laser diode andan optical waveguide. The DBR type laser diode is provided as a lightsource of a fundamental wave, and emits a laser beam having thewavelength of 1,100 nm. The optical waveguide is provided as awavelength conversion element, which is made of a nonlinear opticalmaterial and has periodic domain-inverted structure. In the SHG laserunit 256, the wavelength of the laser beam emitted from the DBR typelaser diode is reduced in half by the optical waveguide, and the laserbeam 255 having the wavelength of 550 nm is generated.

[0312] The laser beam 255 is collimated by the collimator lens 257, andthe collimated laser beam 255 enters the beam splitter 252, in which thecollimated laser beam 255 is reflected toward the condenser lens 251.Then, the collimated and reflected laser beam 255 is condensed by thecondenser lens 251, and enters the optical fiber 253 together with thelaser beam 210 from the laser diode 211.

[0313] The optical fiber 253 has basically the same construction as theoptical fiber 213 in the twelfth embodiment, except that the lightentrance end surface 253 a and the light exit end surface 253 b of theoptical fiber 253 are coated to be antireflective (AR) at the abovewavelengths of 420 nm and 550 nm.

[0314] The laser beam 210 excites Ho³⁺ in the core of the optical fiber253, and fluorescence having the wavelength of 550 nm is generated bythe excitation of Ho³⁺, in the same manner as the twelfth embodiment.Since the wavelength of the above fluorescence is the same as thewavelength of the laser beam 255 from the SHG laser unit 256, the laserbeam 255 is amplified in the optical fiber 253 by receiving the energyof the fluorescence, and the amplified laser beam 255′ is output throughthe light exit end surface 253 b to the forward side of the fiber laseramplifier of FIG. 5.

[0315] Actually, the applicants have obtained the amplified laser beam255′ with the output power of 60 mW when the output power of the SHGlaser unit 256 is 1 mW.

[0316] In addition, when a function of modulation of the laser beam 255is provided to the DBR type laser diode in the SHG laser unit 256, it ispossible to modulate the laser beam 255′.

[0317] Further, the Ho³⁺-doped core of the optical fiber 253 cangenerate the fluorescence having the wavelength of 750 nm by thetransition from ⁵S₂ to ⁵I₇. Therefore, when the coatings applied to thelight entrance end surface 253 a and the light exit end surface 253 b ofthe optical fiber 253 are appropriately arranged, the fiber laseramplifier as the eighteenth embodiment of the present invention canamplify a laser beam having the wavelength of 750 nm.

Nineteenth Embodiment

[0318] The nineteenth embodiment of the present invention is explainedbelow. Since the construction of the fiber laser amplifier as thenineteenth embodiment of the present invention has the same constructionas the eighteenth embodiment except for the portions of the constructionexplained below, the reference numerals in FIG. 5 are also referred toin the following explanations of the nineteenth embodiment.

[0319] The fiber laser amplifier as the nineteenth embodiment isdifferent from the fiber laser amplifier as the eighteenth embodiment inthe rare-earth ion with which the core of the optical fiber 253 is dopedand the coatings applied to the light entrance end surface 253 a and thelight exit end surface 253 b of the optical fiber 253.

[0320] That is, in the nineteenth embodiment, the core of the opticalfiber 253 is doped with 1 atomic percent (atm %) of Sm³⁺ instead ofHo³⁺. In addition, the laser diode 211 used in the nineteenth embodimentemits a laser beam having the wavelength of 404 nm. Further, the lightentrance end surface 253 a and the light exit end surface 253 b of theoptical fiber 253 are coated to be antireflective (AR) at thewavelengths of 566 nm (the wavelength of the fluorescence generated bythe transition from ⁴G_(5/2) to ⁶H_(5/2) in the core of the opticalfiber 253) and 404 nm (the wavelength of the excitation laser beam 210).

[0321] In the fiber laser amplifier as the nineteenth embodiment, theapplicants have obtained the amplified laser beam 255′ from the opticalfiber 253 with the output power of 100 mW when the output power of theSHG laser unit 256 is 1.5 mW.

[0322] Alternatively, the coatings applied to the light entrance endsurface 253 a and the light exit end surface 253 b of the optical fiber253 may be arranged so that a laser beam having a wavelength of 615 or650 nm is amplified in the optical fiber 253, since the Sm³⁺-doped coreof the optical fiber 253 can generate the fluorescence having thewavelength of 615 nm by the transition from ⁴G_(5/2) to ⁶H_(7/2) and thefluorescence having the wavelength of 650 nm by the transition from⁴F_(3/2) to ⁶H_(11/2).

Twentieth Embodiment

[0323] The twentieth embodiment of the present invention is explainedbelow. Since the construction of the fiber laser amplifier as thetwentieth embodiment of the present invention also has the sameconstruction as the eighteenth embodiment except for the portions of theconstruction explained below, the reference numerals in FIG. 5 are alsoreferred to in the following explanations of the twentieth embodiment.

[0324] The fiber laser amplifier as the twentieth embodiment isdifferent from the fiber laser amplifier as the eighteenth embodiment inthe rare-earth ion with which the core of the optical fiber 253 is dopedand the coatings applied to the light entrance end surface 253 a and thelight exit end surface 253 b of the optical fiber 253.

[0325] That is, in the twentieth embodiment, the core of the opticalfiber 253 is doped with 1 atomic percent (atm %) of Eu³⁺ instead ofHo³⁺. In addition, the laser diode 211 used in the twentieth embodimentemits a laser beam having the wavelength of 394 nm. Further, the lightentrance end surface 253 a and the light exit end surface 253 b of theoptical fiber 253 are coated to be antireflective (AR) at thewavelengths of 589 nm (the wavelength of the fluorescence generated bythe transition from ⁵D₀ to ⁷F₂ in the core of the optical fiber 253) and394 nm (the wavelength of the excitation laser beam 210).

[0326] In the fiber laser amplifier as the twentieth embodiment, theapplicants have obtained the amplified laser beam 255′ from the opticalfiber 253 with the output power of 50 mW when the output power of theSHG laser unit 256 is 1 mW.

Twenty-First Embodiment

[0327] The twenty-first embodiment of the present invention is explainedbelow. Since the construction of the fiber laser amplifier as thetwenty-first embodiment of the present invention has the sameconstruction as the eighteenth embodiment except for the portions of theconstruction explained below, the reference numerals in FIG. 5 are alsoreferred to in the following explanations of the twenty-firstembodiment.

[0328] The fiber laser amplifier as the twenty-first embodiment isdifferent from the fiber laser amplifier as the eighteenth embodiment inthe rare-earth ion with which the core of the optical fiber 253 is dopedand the coatings applied to the light entrance end surface 253 a and thelight exit end surface 253 b of the optical fiber 253.

[0329] That is, in the twenty-first embodiment, the core of the opticalfiber 253 is doped with 1 atomic percent (atm %) of Dy³⁺ instead ofHo³⁺. In addition, the laser diode 211 used in the twenty-firstembodiment emits a laser beam having the wavelength of 390 nm. Further,the light entrance end surface 253 a and the light exit end surface 253b of the optical fiber 253 are coated to be antireflective (AR) at thewavelengths of 572 nm (the wavelength of the fluorescence generated bythe transition from ⁴F_(9/2) to ⁶H_(13/2) in the core of the opticalfiber 253) and 390 nm (the wavelength of the excitation laser beam 210).In the fiber laser amplifier as the twenty-first embodiment, theapplicants have obtained the amplified laser beam 255′ from the opticalfiber 253 with the output power of 80 mW when the output power of theSHG laser unit 256 is 1.5 mW.

[0330] Alternatively, the coatings applied to the light entrance endsurface 253 a and the light exit end surface 253 b of the optical fiber253 may be arranged so that a laser beam having the wavelength of 664 nmis amplified in the optical fiber 253, since the Dy³⁺-doped core of theoptical fiber 253 can generate the fluorescence having the wavelength of664 nm by the transition from ⁴F_(9/2) to ⁶H_(11/2).

Twenty-Second Embodiment

[0331] The twenty-second embodiment of the present invention isexplained below. Since the construction of the fiber laser amplifier asthe twenty-second embodiment of the present invention has the sameconstruction as the eighteenth embodiment except for the portions of theconstruction explained below, the reference numerals in FIG. 5 are alsoreferred to in the following explanations of the twenty-secondembodiment.

[0332] The fiber laser amplifier as the twenty-second embodiment isdifferent from the fiber laser amplifier as the eighteenth embodiment inthe rare-earth ion with which the core of the optical fiber 253 is dopedand the coatings applied to the light entrance end surface 253 a and thelight exit end surface 253 b of the optical fiber 253.

[0333] That is, in the twenty-second embodiment, the core of the opticalfiber 253 is doped with 1 atomic percent (atm %) of Er³⁺ instead ofHo³⁺. In addition, the laser diode 211 used in the twenty-secondembodiment emits a laser beam having the wavelength of 406 nm. Further,the light entrance end surface 253 a and the light exit end surface 253b of the optical fiber 253 are coated to be antireflective (AR) at thewavelengths of 554 nm (the wavelength of the fluorescence generated bythe transition from ²H_(9/2) to ⁴I_(13/2) in the core of the opticalfiber 253) and 406 nm (the wavelength of the excitation laser beam 210).

[0334] In the fiber laser amplifier as the twenty-second embodiment, theapplicants have obtained the amplified laser beam 255′ from the opticalfiber 253 with the output power of 80 mW when the output power of theSHG laser unit 256 is 1 mW.

[0335] Alternatively, the coatings applied to the light entrance endsurface 253 a and the light exit end surface 253 b of the optical fiber253 may be arranged so that a laser beam having the wavelength of 540 nmis amplified in the optical fiber 253, since the Er³⁺-doped core of theoptical fiber 253 can generate the fluorescence having the wavelength of540 nm by the transition from ⁴S_(3/2) to ⁴I_(15/2).

[0336] In addition, the excitation wavelength may be 380 nm instead of406 nm.

Twenty-Third Embodiment

[0337] The twenty-third embodiment of the present invention is explainedbelow. Since the construction of the fiber laser amplifier as thetwenty-third embodiment of the present invention also has the sameconstruction as the eighteenth embodiment except for the portions of theconstruction explained below, the reference numerals in FIG. 5 are alsoreferred to in the following explanations of the twenty-thirdembodiment.

[0338] The fiber laser amplifier as the twenty-third embodiment isdifferent from the fiber laser amplifier as the eighteenth embodiment inthe rare-earth ion with which the core of the optical fiber 253 is dopedand the coatings applied to the light entrance end surface 253 a and thelight exit end surface 253 b of the optical fiber 253.

[0339] That is, in the twenty-third embodiment, the core of the opticalfiber 253 is doped with 1 atomic percent (atm %) of Tb³⁺ instead ofHo³⁺. In addition, the laser diode 211 used in the twenty-thirdembodiment emits a laser beam having the wavelength of 380 nm. Further,the light entrance end surface 253 a and the light exit end surface 253b of the optical fiber 253 are coated to be antireflective (AR) at thewavelengths of 540 nm (the wavelength of the fluorescence generated bythe transition from ⁵D₄ to ⁷F₅ in the core of the optical fiber 253) and380 nm (the wavelength of the excitation laser beam 210).

[0340] In the fiber laser amplifier as the twenty-third embodiment, theapplicants have obtained the amplified laser beam 255′ from the opticalfiber 253 with the output power of 70 mW when the output power of theSHG laser unit 256 is 1.5 mW.

Variations of Twelfth to Twenty-Third Embodiments

[0341] The active layers of the laser diodes used as excitation lightsources in the constructions of the twelfth to twenty-third embodimentscan be made of an InGaN-based, InGaNAs-based, or GaNAs-based compoundmaterial. In particular, when an absorption band of a solid-state lasercrystal is located on the longer wavelength side of the outputwavelength of the laser diode, it is preferable to use the InGaNAs-basedor GaNAs-based compound material, since the wavelength of the laseroscillation in the InGaNAs-based or GaNAs-based compound laser diode canbe lengthened more easily than that in the InGaN-based compound laserdiode. That is, the absorption efficiency in the InGaNAs-based orGaNAs-based compound laser diode can be enhanced more easily than thatin the InGaN-based compound laser diode.

What is claimed is:
 1. A laser-diode-excited solid-state laser apparatuscomprising: a GaN-based compound laser diode which emits an excitationlaser beam; and a solid-state laser crystal which is doped with Ho³⁺,and emits a solid-state laser beam generated by one of a firsttransition from ⁵S₂ to ⁵I₇ and a second transition from ⁵S₂ to ⁵I₈ whenthe solid-state laser crystal is excited with said excitation laserbeam.
 2. A laser-diode-excited solid-state laser apparatus according toclaim 1, wherein said solid-state laser beam is generated by said firsttransition from ⁵S₂ to ⁵I₇ and is in a wavelength range of 740 to 760nm.
 3. A laser-diode-excited solid-state laser apparatus according toclaim 1, wherein said solid-state laser beam is generated by said secondtransition from ⁵S₂ to ⁵I₈ and is in a wavelength range of 540 to 560nm.
 4. A laser-diode-excited solid-state laser apparatus according toclaim 1, wherein said solid-state laser crystal is doped with norare-earth ion other than Ho³⁺.
 5. A laser-diode-excited solid-statelaser apparatus according to claim 1, wherein said GaN-based compoundlaser diode has an active layer made of one of InGaN, InGaNAs, and GaNAsmaterials.
 6. A laser-diode-excited solid-state laser apparatuscomprising: a GaN-based compound laser diode which emits an excitationlaser beam; and a solid-state laser crystal which is doped with Sm³⁺,and emits a solid-state laser beam generated by one of a firsttransition from ⁴G_(5/2) to ⁶H_(5/2), a second transition from ⁴G_(5/2)to ⁶H_(7/2), and a third transition from ⁴F_(3/2) to ⁶H_(11/2) when thesolid-state laser crystal is excited with said excitation laser beam. 7.A laser-diode-excited solid-state laser apparatus according to claim 6,wherein said solid-state laser beam is generated by said firsttransition from ⁴G_(5/2) to ⁶H_(5/2) and is in a wavelength range of 556to 576 nm.
 8. A laser-diode-excited solid-state laser apparatusaccording to claim 6, wherein said solid-state laser beam is generatedby said second transition from ⁴G_(5/2) to ⁶H_(7/2) and is in awavelength range of 605 to 625 nm.
 9. A laser-diode-excited solid-statelaser apparatus according to claim 6, wherein said solid-state laserbeam is generated by said third transition from ⁴F_(3/2) to ⁶H_(11/2)and is in a wavelength range of 640 to 660 nm.
 10. A laser-diode-excitedsolid-state laser apparatus according to claim 6, wherein saidsolid-state laser crystal is doped with no rare-earth ion other thanSm³⁺.
 11. A laser-diode-excited solid-state laser apparatus according toclaim 6, wherein said GaN-based compound laser diode has an active layermade of one of InGaN, InGaNAs, and GaNAs materials.
 12. Alaser-diode-excited solid-state laser apparatus comprising: a GaN-basedcompound laser diode which emits an excitation laser beam; and asolid-state laser crystal which is doped with Eu³⁺, and emits asolid-state laser beam generated by a transition from ⁵D₀ to ⁷F₂ whenthe solid-state laser crystal is excited with said excitation laserbeam.
 13. A laser-diode-excited solid-state laser apparatus according toclaim 12, wherein said solid-state laser beam is in a wavelength rangeof 579 to 599 nm.
 14. A laser-diode-excited solid-state laser apparatusaccording to claim 12, wherein said solid-state laser crystal is dopedwith no rare-earth ion other than Eu³⁺.
 15. A laser-diode-excitedsolid-state laser apparatus according to claim 12, wherein saidGaN-based compound laser diode has an active layer made of one of InGaN,InGaNAs, and GaNAs materials.
 16. A laser-diode-excited solid-statelaser apparatus comprising: a GaN-based compound laser diode which emitsan excitation laser beam; and a solid-state laser crystal which is dopedwith Dy³⁺, and emits a solid-state laser beam generated by one of afirst transition from ⁴F_(9/2) to ⁶H_(13/2) and a second transition from⁴F_(9/2) to ⁶H_(11/2) when the solid-state laser crystal is excited withsaid excitation laser beam.
 17. A laser-diode-excited solid-state laserapparatus according to claim 16, wherein said solid-state laser beam isgenerated by said first transition from ⁴F_(9/2) to ⁶H_(13/2) and is ina wavelength range of 562 to 582 nm.
 18. A laser-diode-excitedsolid-state laser apparatus according to claim 16, wherein saidsolid-state laser beam is generated by said second transition from⁴F_(9/2) to ⁶H_(11/2) and is in a wavelength range of 654 to 674 nm. 19.A laser-diode-excited solid-state laser apparatus according to claim 16,wherein said solid-state laser crystal is doped with no rare-earth ionother than Dy³⁺.
 20. A laser-diode-excited solid-state laser apparatusaccording to claim 16, wherein said GaN-based compound laser diode hasan active layer made of one of InGaN, InGaNAs, and GaNAs materials. 21.A laser-diode-excited solid-state laser apparatus comprising: aGaN-based compound laser diode which emits an excitation laser beam; anda solid-state laser crystal which is doped with Er³⁺, and emits asolid-state laser beam generated by one of a first transition from⁴S_(3/2) to ⁴I_(15/2) and a second transition from ²H_(9/2) to ⁴I_(13/2)when the solid-state laser crystal is excited with said excitation laserbeam.
 22. A laser-diode-excited solid-state laser apparatus according toclaim 21, wherein said solid-state laser beam is generated by said firsttransition from ⁴S_(3/2) to ⁴I_(15/2) and is in a wavelength range of530 to 550 nm.
 23. A laser-diode-excited solid-state laser apparatusaccording to claim 21, wherein said solid-state laser beam is generatedby said second transition from ²H_(9/2) to ⁴I_(13/2) and is in awavelength range of 544 to 564 nm.
 24. A laser-diode-excited solid-statelaser apparatus according to claim 21, wherein said solid-state lasercrystal is doped with no rare-earth ion other than Er³⁺.
 25. Alaser-diode-excited solid-state laser apparatus according to claim 21,wherein said GaN-based compound laser diode has an active layer made ofone of InGaN, InGaNAs, and GaNAs materials.
 26. A laser-diode-excitedsolid-state laser apparatus comprising: a GaN-based compound laser diodewhich emits an excitation laser beam; and a solid-state laser crystalwhich is doped with Tb³⁺, and emits a solid-state laser beam generatedby a transition from ⁵D₄ to ⁷F₅ when the solid-state laser crystal isexcited with said excitation laser beam.
 27. A laser-diode-excitedsolid-state laser apparatus according to claim 26, wherein saidsolid-state laser beam is in a wavelength range of 530 to 550 nm.
 28. Alaser-diode-excited solid-state laser apparatus according to claim 26,wherein said solid-state laser crystal is doped with no rare-earth ionother than Tb³⁺.
 29. A laser-diode-excited solid-state laser apparatusaccording to claim 26, wherein said GaN-based compound laser diode hasan active layer made of one of InGaN, InGaNAs, and GaNAs materials. 30.A laser-diode-excited solid-state laser apparatus comprising: a laserdiode which has an active layer made of one of InGaN, InGaNAs, and GaNAsmaterials, and emits an excitation laser beam; a solid-state lasercrystal which is doped with at least one rare-earth ion including Ho³⁺,and emits a solid-state laser beam generated by one of a firsttransition from ⁵S₂ to ⁵I₇ and a second transition from ⁵S₂ to ⁵I₈ whenthe solid-state laser crystal is excited with said excitation laserbeam; and an optical wavelength conversion element which converts saidsolid-state laser beam into ultraviolet laser light by wavelengthconversion.
 31. A laser-diode-excited solid-state laser apparatusaccording to claim 30, wherein said solid-state laser beam is generatedby said first transition from ⁵S₂ to ⁵I₇ and has a wavelength of about750 nm, and said ultraviolet laser light has a wavelength of about 375nm.
 32. A laser-diode-excited solid-state laser apparatus according toclaim 30, wherein said solid-state laser beam is generated by saidsecond transition from ⁵S₂ to ⁵I₈ and has a wavelength of about 550 nm,and said ultraviolet laser light has a wavelength of about 275 nm.
 33. Alaser-diode-excited solid-state laser apparatus according to claim 30,wherein said solid-state laser crystal is doped with no rare-earth ionother than Ho³⁺.
 34. A laser-diode-excited solid-state laser apparatusaccording to claim 30, wherein said optical wavelength conversionelement is realized by a nonlinear optical crystal having a periodicdomain-inverted structure.
 35. A laser-diode-excited solid-state laserapparatus comprising: a laser diode which has an active layer made ofone of InGaN, InGaNAs, and GaNAs materials, and emits an excitationlaser beam; a solid-state laser crystal which is doped with at least onerare-earth ion including Sm³⁺, and emits a solid-state laser beamgenerated by one of a first transition from ⁴G_(5/2) to ⁶H_(5/2), asecond transition from ⁴G_(5/2) to ⁶H_(7/2), and a third transition from⁴F_(3/2) to ⁶H_(11/2) when the solid-state laser crystal is excited withsaid excitation laser beam; and an optical wavelength conversion elementwhich converts said solid-state laser beam into ultraviolet laser lightby wavelength conversion.
 36. A laser-diode-excited solid-state laserapparatus according to claim 35, wherein said solid-state laser beam isgenerated by said first transition from ⁴G_(5/2) to ⁶H_(5/2) and has awavelength of about 566 nm, and said ultraviolet laser light has awavelength of about 283 nm.
 37. A laser-diode-excited solid-state laserapparatus according to claim 35, wherein said solid-state laser beam isgenerated by said second transition from ⁴G_(5/2) to ⁶H_(7/2) and has awavelength of about 615 nm, and said ultraviolet laser light has awavelength of about 308 nm.
 38. A laser-diode-excited solid-state laserapparatus according to claim 35, wherein said solid-state laser beam isgenerated by said third transition from ⁴F_(3/2) to ⁶H_(11/2) and has awavelength of about 650 nm, and said ultraviolet laser light has awavelength of about 325 nm.
 39. A laser-diode-excited solid-state laserapparatus according to claim 35, wherein said solid-state laser crystalis doped with no rare-earth ion other than Sm³⁺.
 40. Alaser-diode-excited solid-state laser apparatus according to claim 35,wherein said optical wavelength conversion element is realized by anonlinear optical crystal having a periodic domain-inverted structure.41. A laser-diode-excited solid-state laser apparatus comprising: alaser diode which has an active layer made of one of InGaN, InGaNAs, andGaNAs materials, and emits an excitation laser beam; a solid-state lasercrystal which is doped with at least one rare-earth ion including Eu³⁺,and emits a solid-state laser beam generated by a transition from ⁵D₀ to⁷F₂ when the solid-state laser crystal is excited with said excitationlaser beam; and an optical wavelength conversion element which convertssaid solid-state laser beam into ultraviolet laser light by wavelengthconversion.
 42. A laser-diode-excited solid-state laser apparatusaccording to claim 41, wherein said solid-state laser beam has awavelength of about 589 nm, and said ultraviolet laser light has awavelength of about 295 nm.
 43. A laser-diode-excited solid-state laserapparatus according to claim 41, wherein said solid-state laser crystalis doped with no rare-earth ion other than Eu³⁺.
 44. Alaser-diode-excited solid-state laser apparatus according to claim 41,wherein said optical wavelength conversion element is realized by anonlinear optical crystal having a periodic domain-inverted structure.45. A laser-diode-excited solid-state laser apparatus comprising: alaser diode which has an active layer made of one of InGaN, InGaNAs, andGaNAs materials, and emits an excitation laser beam; a solid-state lasercrystal which is doped with at least one rare-earth ion including Dy³⁺,and emits a solid-state laser beam generated by one of a firsttransition from ⁴F_(9/2) to ⁶H_(13/2) and a second transition from⁴F_(9/2) to ⁶H_(11/2) when the solid-state laser crystal is excited withsaid excitation laser beam; and an optical wavelength conversion elementwhich converts said solid-state laser beam into ultraviolet laser lightby wavelength conversion.
 46. A laser-diode-excited solid-state laserapparatus according to claim 45, wherein said solid-state laser beam isgenerated by said first transition from ⁴F_(9/2) to ⁶H_(13/2) and has awavelength of about 572 nm, and said ultraviolet laser light has awavelength of about 286 nm.
 47. A laser-diode-excited solid-state laserapparatus according to claim 45, wherein said solid-state laser beam isgenerated by said second transition from ⁴F_(9/2) to ⁶H_(11/2) and has awavelength of about 664 nm, and said ultraviolet laser light has awavelength of about 332 nm.
 48. A laser-diode-excited solid-state laserapparatus according to claim 45, wherein said solid-state laser crystalis doped with no rare-earth ion other than Dy³⁺.
 49. Alaser-diode-excited solid-state laser apparatus according to claim 45,wherein said optical wavelength conversion element is realized by anonlinear optical crystal having a periodic domain-inverted structure.50. A laser-diode-excited solid-state laser apparatus comprising: alaser diode which has an active layer made of one of InGaN, InGaNAs, andGaNAs materials, and emits an excitation laser beam; a solid-state lasercrystal which is doped with at least one rare-earth ion including Er³⁺,and emits a solid-state laser beam generated by one of a firsttransition from ⁴S_(3/2) to ⁴I_(15/2) and a second transition from²H_(9/2) to ⁴I_(13/2) when the solid-state laser crystal is excited withsaid excitation laser beam; and an optical wavelength conversion elementwhich converts said solid-state laser beam into ultraviolet laser lightby wavelength conversion.
 51. A laser-diode-excited solid-state laserapparatus according to claim 50, wherein said solid-state laser beam isgenerated by said first transition from ⁴S_(3/2) to ⁴I_(15/2) and has awavelength of about 540 nm, and said ultraviolet laser light has awavelength of about 270 nm.
 52. A laser-diode-excited solid-state laserapparatus according to claim 50, wherein said solid-state laser beam isgenerated by said second transition from ²H_(9/2) to ⁴I_(13/2) and has awavelength of about 554 nm, and said ultraviolet laser light has awavelength of about 277 nm.
 53. A laser-diode-excited solid-state laserapparatus according to claim 50, wherein said solid-state laser crystalis doped with no rare-earth ion other than Er³⁺.
 54. Alaser-diode-excited solid-state laser apparatus according to claim 50,wherein said optical wavelength conversion element is realized by anonlinear optical crystal having a periodic domain-inverted structure.55. A fiber laser apparatus comprising: a GaN-based compound laser diodewhich emits a first laser beam; and an optical fiber which has a coredoped with Ho³⁺, and emits a second laser beam generated by one of afirst transition from ⁵S₂ to ⁵I₇ and a second transition from ⁵S₂ to ⁵I₈when the optical fiber is excited with said first laser beam.
 56. Afiber laser apparatus according to claim 55, wherein said second laserbeam is generated by said first transition from ⁵S₂ to ⁵I₇ and is in awavelength range of 740 to 760 nm.
 57. A fiber laser apparatus accordingto claim 55, wherein said second laser beam is generated by said secondtransition from ⁵S₂ to ⁵I₈ and is in a wavelength range of 540 to 560nm.
 58. A fiber laser apparatus according to claim 55, wherein said coreof said optical fiber is doped with no rare-earth ion other than Ho³⁺.59. A fiber laser apparatus according to claim 55, wherein saidGaN-based compound laser diode has an active layer made of one of InGaN,InGaNAs, and GaNAs materials.
 60. A fiber laser apparatus comprising: aGaN-based compound laser diode which emits a first laser beam; and anoptical fiber which has a core doped with Sm³⁺, and emits a second laserbeam generated by one of a first transition from ⁴G_(5/2) to ⁶H_(5/2), asecond transition from ⁴G_(5/2) to ⁶H_(7/2), and a third transition from⁴F_(3/2) to ⁶H_(11/2) when the optical fiber is excited with said firstlaser beam.
 61. A fiber laser apparatus according to claim 60, whereinsaid second laser beam is generated by said first transition from⁴G_(5/2) to ⁶H_(5/2) and is in a wavelength range of 556 to 576 nm. 62.A fiber laser apparatus according to claim 60, wherein said second laserbeam is generated by said second transition from ⁴G_(5/2) to ⁶H_(7/2)and is in a wavelength range of 605 to 625 nm.
 63. A fiber laserapparatus according to claim 60, wherein said second laser beam isgenerated by said third transition from ⁴F_(3/2) to ⁶H_(11/2) and is ina wavelength range of 640 to 660 nm.
 64. A fiber laser apparatusaccording to claim 60, wherein said core of said optical fiber is dopedwith no rare-earth ion other than Sm³⁺.
 65. A fiber laser apparatusaccording to claim 60, wherein said GaN-based compound laser diode hasan active layer made of one of InGaN, InGaNAs, and GaNAs materials. 66.A fiber laser apparatus comprising: a GaN-based compound laser diodewhich emits a first laser beam; and an optical fiber which has a coredoped with Eu³⁺, and emits a second laser beam generated by a transitionfrom ⁵D₀ to ⁷F₂ when the optical fiber is excited with said first laserbeam.
 67. A fiber laser apparatus according to claim 66, wherein saidsecond laser beam is in a wavelength range of 579 to 599 nm.
 68. A fiberlaser apparatus according to claim 66, wherein said core of said opticalfiber is doped with no rare-earth ion other than Eu³⁺.
 69. A fiber laserapparatus according to claim 66, wherein said GaN-based compound laserdiode has an active layer made of one of InGaN, InGaNAs, and GaNAsmaterials.
 70. A fiber laser apparatus comprising: a GaN-based compoundlaser diode which emits a first laser beam; and an optical fiber whichhas a core doped with Dy³⁺, and emits a second laser beam generated byone of a first transition from ⁴F_(9/2) to ⁶H_(13/2) and a secondtransition from ⁴F_(9/2) to ⁶H_(11/2) when the optical fiber is excitedwith said first laser beam.
 71. A fiber laser apparatus according toclaim 70, wherein said second laser beam is generated by said firsttransition from ⁴F_(9/2) to ⁶H_(13/2) and is in a wavelength range of562 to 582 nm.
 72. A fiber laser apparatus according to claim 70,wherein said second laser beam is generated by said second transitionfrom ⁴F_(9/2) to ⁶H_(11/2) and is in a wavelength range of 654 to 674nm.
 73. A fiber laser apparatus according to claim 70, wherein said coreof said optical fiber is doped with no rare-earth ion other than Dy³⁺.74. A fiber laser apparatus according to claim 70, wherein saidGaN-based compound laser diode has an active layer made of one of InGaN,InGaNAs, and GaNAs materials.
 75. A fiber laser apparatus comprising: aGaN-based compound laser diode which emits a first laser beam; and anoptical fiber which has a core doped with Er³⁺, and emits a second laserbeam generated by one of a first transition from ⁴S_(3/2) to ⁴I_(15/2)and a second transition from ²H_(9/2) to ⁴I_(13/2) when the opticalfiber is excited with said first laser beam.
 76. A fiber laser apparatusaccording to claim 75, wherein said second laser beam is generated bysaid first transition from ⁴S_(3/2) to ⁴I_(15/2) and is in a wavelengthrange of 530 to 550 nm.
 77. A fiber laser apparatus according to claim75, wherein said second laser beam is generated by said secondtransition from ²H_(9/2) to ⁴I_(13/2) and is in a wavelength range of544 to 564 nm.
 78. A fiber laser apparatus according to claim 75,wherein said core of said optical fiber is doped with no rare-earth ionother than Er³⁺.
 79. A fiber laser apparatus according to claim 75,wherein said GaN-based compound laser diode has an active layer made ofone of InGaN, InGaNAs, and GaNAs materials.
 80. A fiber laser apparatuscomprising: a GaN-based compound laser diode which emits a first laserbeam; and an optical fiber which has a core doped with Tb³⁺, and emits asecond laser beam generated by a transition from ⁵D₄ to ⁷F₅ when theoptical fiber is excited with said first laser beam.
 81. A fiber laserapparatus according to claim 80, wherein said second laser beam is in awavelength range of 530 to 550 nm.
 82. A fiber laser apparatus accordingto claim 80, wherein said core of said optical fiber is doped with norare-earth ion other than Tb³⁺.
 83. A fiber laser apparatus according toclaim 80, wherein said GaN-based compound laser diode has an activelayer made of one of InGaN, InGaNAs, and GaNAs materials.
 84. A fiberlaser amplifier comprising: a GaN-based compound laser diode which emitsan excitation laser beam; and an optical fiber which has a core dopedwith Ho³⁺, and amplifies incident light which has a wavelength within awavelength range of fluorescence generated by one of a first transitionfrom ⁵S₂ to ⁵I₇ and a second transition from ⁵S₂ to ⁵I₈ when the opticalfiber is excited with said excitation laser beam.
 85. A fiber laseramplifier according to claim 85, wherein said fluorescence is generatedby said first transition from ⁵S₂ to ⁵I₇ and is in a wavelength range of740 to 760 nm.
 86. A fiber laser amplifier according to claim 84,wherein said fluorescence is generated by said second transition from⁵S₂ to ⁵I₈ and is in a wavelength range of 540 to 560 nm.
 87. A fiberlaser amplifier according to claim 84, wherein said core of said opticalfiber is doped with no rare-earth ion other than Ho³⁺.
 88. A fiber laseramplifier according to claim 84, wherein said GaN-based compound laserdiode has an active layer made of one of InGaN, InGaNAs, and GaNAsmaterials.
 89. A fiber laser amplifier comprising: a GaN-based compoundlaser diode which emits an excitation laser beam; and an optical fiberwhich has a core doped with Sm³⁺, and amplifies incident light which hasa wavelength within a wavelength range of fluorescence generated by oneof a first transition from ⁴G_(5/2) to ⁶H_(5/2), a second transitionfrom ⁴G_(5/2) to ⁶H_(7/2), and a third transition from ⁴F_(3/2) to⁶H_(11/2) when the optical fiber is excited with said excitation laserbeam.
 90. A fiber laser amplifier according to claim 89, wherein saidfluorescence is generated by said first transition from ⁴G_(5/2) to⁶H_(5/2) and is in a wavelength range of 556 to 576 nm.
 91. A fiberlaser amplifier according to claim 89, wherein said fluorescence isgenerated by said second transition from ⁴G_(5/2) to ⁶H_(7/2) and is ina wavelength range of 605 to 625 nm.
 92. A fiber laser amplifieraccording to claim 89, wherein said fluorescence is generated by saidthird transition from ⁴F_(3/2) to ⁶H_(11/2) and is in a wavelength rangeof 640 to 660 nm.
 93. A fiber laser amplifier according to claim 89,wherein said core of said optical fiber is doped with no rare-earth ionother than Sm³⁺.
 94. A fiber laser amplifier according to claim 89,wherein said GaN-based compound laser diode has an active layer made ofone of InGaN, InGaNAs, and GaNAs materials.
 95. A fiber laser amplifiercomprising: a GaN-based compound laser diode which emits an excitationlaser beam; and an optical fiber which has a core doped with Eu³⁺, andamplifies incident light which has a wavelength within a wavelengthrange of fluorescence generated by a transition from ⁵D₀ to ⁷F₂ when theoptical fiber is excited with said excitation laser beam.
 96. A fiberlaser amplifier according to claim 95, wherein said fluorescence is in awavelength range of 579 to 599 nm.
 97. A fiber laser amplifier accordingto claim 95, wherein said core of said optical fiber is doped with norare-earth ion other than Eu³⁺.
 98. A fiber laser amplifier according toclaim 95, wherein said GaN-based compound laser diode has an activelayer made of one of InGaN, InGaNAs, and GaNAs materials.
 99. A fiberlaser amplifier comprising: a GaN-based compound laser diode which emitsan excitation laser beam; and an optical fiber which has a core dopedwith Dy³⁺, and amplifies incident light which has a wavelength within awavelength range of fluorescence generated by one of a first transitionfrom ⁴F_(9/2) to ⁶H_(13/2) and a second transition from ⁴F_(9/2) to⁶H_(11/2) when the optical fiber is excited with said excitation laserbeam.
 100. A fiber laser amplifier according to claim 99, wherein saidfluorescence is generated by said first transition from ⁴F_(9/2) to⁶H_(13/2) and is in a wavelength range of 562 to 582 nm.
 101. A fiberlaser amplifier according to claim 99, wherein said fluorescence isgenerated by said second transition from ⁴F_(9/2) to ⁶H_(11/2) and is ina wavelength range of 654 to 674 nm.
 102. A fiber laser amplifieraccording to claim 99, wherein said core of said optical fiber is dopedwith no rare-earth ion other than Dy³⁺.
 103. A fiber laser amplifieraccording to claim 99, wherein said GaN-based compound laser diode hasan active layer made of one of InGaN, InGaNAs, and GaNAs materials. 104.A fiber laser amplifier comprising: a GaN-based compound laser diodewhich emits an excitation laser beam; and an optical fiber which has acore doped with Er³⁺, and amplifies incident light which has awavelength within a wavelength range of fluorescence generated by one ofa first transition from ⁴S_(3/2) to ⁴I_(15/2) and a second transitionfrom ²H_(9/2) to ⁴I_(13/2) when the optical fiber is excited with saidexcitation laser beam.
 105. A fiber laser amplifier according to claim104, wherein said fluorescence is generated by said first transitionfrom ⁴S_(3/2) to ⁴I_(15/2) and is in a wavelength range of 530 to 550nm.
 106. A fiber laser amplifier according to claim 104, wherein saidfluorescence is generated by said second transition from ²H_(9/2) to⁴I_(13/2) and is in a wavelength range of 544 to 564 nm.
 107. A fiberlaser amplifier according to claim 104, wherein said core of saidoptical fiber is doped with no rare-earth ion other than Er³⁺.
 108. Afiber laser amplifier according to claim 104, wherein said GaN-basedcompound laser diode has an active layer made of one of InGaN, InGaNAs,and GaNAs materials.
 109. A fiber laser amplifier comprising: aGaN-based compound laser diode which emits an excitation laser beam; andan optical fiber which has a core doped with Tb³⁺, and amplifiesincident light which has a wavelength within a wavelength range offluorescence generated by a transition from ⁵D₄ to ⁷F₅ when the opticalfiber is excited with said excitation laser beam.
 110. A fiber laseramplifier according to claim 109, wherein said fluorescence is in awavelength range of 530 to 550 nm.
 111. A fiber laser amplifieraccording to claim 109, wherein said core of said optical fiber is dopedwith no rare-earth ion other than Tb³⁺.
 112. A fiber laser amplifieraccording to claim 109, wherein said GaN-based compound laser diode hasan active layer made of one of InGaN, InGaNAs, and GaNAs materials.